[Odsh]

When a character creates an effect that lasts for 1 combat round, when exactly does the effect end:

• At the end of the current combat round?
• At the end of the next combat round?
• The next combat round just before that character's next turn?
• The next combat round just before when that character would have its next turn if its Initiative didn't change until then?
• other

The logical answer would be the fourth one, but there may be a simpler rule somewhere that I oversaw.

[Xenon]

If you activate something during your character's Player Turn that will last 1 Combat Round then I don't see why it would not end just before your character's next Player Turn in the next Combat Round...

[Stainless Steel Devil Rat]

When a character creates an effect that lasts for 1 combat round, when exactly does the effect end:

• At the end of the current combat round?
• At the end of the next combat round?
• The next combat round just before that character's next turn?
• The next combat round just before when that character would have its next turn if its Initiative didn't change until then?
• other

The logical answer would be the fourth one, but there may be a simpler rule somewhere that I oversaw.

It depends on the effect, of course.  But generally the answer is going to be option 5: GM Discretion.

Most Statuses, for example, have no duration specified.  When does Wet wear off?  Well, we all have enough real world experience to have a good guess how long that should take in the Sixth World.  Other conditions like Blinded can be reasonably inferred, but not everyone has been tear-gassed or gouged in the eye.  Sometimes it's harder to apply "common sense" without a basis in real world experience. This is why effects with no given duration last as long as the GM says they do. 

[Odsh]

It depends on the effect, of course.  But generally the answer is going to be option 5: GM Discretion.

Most Statuses, for example, have no duration specified.  When does Wet wear off?  Well, we all have enough real world experience to have a good guess how long that should take in the Sixth World.  Other conditions like Blinded can be reasonably inferred, but not everyone has been tear-gassed or gouged in the eye.  Sometimes it's harder to apply "common sense" without a basis in real world experience. This is why effects with no given duration last as long as the GM says they do.

To give a bit of context, I'm setting up a Shadowrun campaign on roll20 for which I created a deck with a card for each possible status effect. The idea is to play a card next to a character when he's affected by that status.
To go a bit further, I want to use the roll20 API to optionally add a "remaining duration" on a status card - provided that the remaining duration is clearly defined of course. For example, when a mage uses counterspelling to protect a character, the protection lasts for (Magic) rounds.
Something along those lines: "Effect will end in X turns at Initiative Y", or "Expired!" when the effect has worn off.
This would be automatically updated as the turn order progresses.
Which is why I'm looking for a clear ruling to write the code that will be executed behind the scene.

[Stainless Steel Devil Rat]

I don't want to say anything that would dissuade anyone from making a 6e mod for Roll20, as Ghost knows we need one

However, I do think you may be going against the intended flow of 6e by codifying things to that degree.  Statuses are intended to have an ambiguous duration so that the GM can extend them due to extenuating circumstances, or shorten them do to mitigating circumstances.  If you and I are affected by a Flash Bang, and you have a Damper and I do not... you should not only suffer less levels of the Deafened status (as per the rule for Dampers) but you should probably also lose whatever Deafened Status you end up with faster than I do with mine, neh?

Totally not official, but here's my personal take on how long statuses last, from longest to shortest: Until the end of the scene, Until the end of the combat, a "few" rounds.  Maybe, in extreme cases, a status might be "permanent" until healed via surgery.

In the case of effects that DO have an explicit duration measured in rounds:  I believe the best practice is to assume a "round" counts as complete when you hit the end of the round.  Unless you have good reason to go otherwise.... I prefer this to "until the end of your next turn" or "until the start of your next turn" because when those are not said, it doesn't necessarily mean that.

You mentioned Counterspell: it lasts a number of rounds = Magic.  Ok, so if you go last, counterspell, and then the round ends, that's already 1 round.  Yes, I mean that.  Why?  Because Counterspell is an Anytime action.  If you are going to counterspell, then just do it at the start of the round before the first turn, rather than waiting until after they cast their spells, neh?

[Odsh]

However, I do think you may be going against the intended flow of 6e by codifying things to that degree.  Statuses are intended to have an ambiguous duration so that the GM can extend them due to extenuating circumstances, or shorten them do to mitigating circumstances.  If you and I are affected by a Flash Bang, and you have a Damper and I do not... you should not only suffer less levels of the Deafened status (as per the rule for Dampers) but you should probably also lose whatever Deafened Status you end up with faster than I do with mine, neh?

You're right, I do not wish to make things more complicated than needed and I could simply rule that things last until the end of the round, the scene, or whatever makes more sense.

But in those cases where a specific amount of rounds is given by the rules, I'd like to try and follow those rules. After all, it makes sense that a mage with 1 Magic can't protect his allies for as long as the experienced Initiate with 10 Magic. The second one will likely protect everyone for the whole combat, the first one probably not. I think that's a case where GM fiat is uncalled for.
Plus this automation I foresee in roll20 should take care of the tedious time tracking part, so it should help with the flow instead of going against it.

You mentioned Counterspell: it lasts a number of rounds = Magic.  Ok, so if you go last, counterspell, and then the round ends, that's already 1 round.  Yes, I mean that.  Why?  Because Counterspell is an Anytime action.  If you are going to counterspell, then just do it at the start of the round before the first turn, rather than waiting until after they cast their spells, neh?

I see what you mean. But even if you counterspell out-of-turn, you still act at someone else's Initiative, no? Or at least just after or just before someone else.

So you could for example rule that one round lasts until the next combat round at that precise same Initiative. Seems complicated if you have to keep track of that yourself, but a program can just memorize the Initiative X of the token on top of the turn tracker when you set the duration and count down the turns each time the Initiative of the Token on top drops from A to B, with A>X>=B.

I agree that the usefulness of this is very situational though.

EDIT: I was thinking about other cases where the Initiative would be more important, like for example a Barghest's Paralyzing Howl. Just to discover that the duration of Paralyzing Howl is missing 
And why does Paralyzing Howl inflict the "Paralyzed" status that is in reality named "Stilled". Just like the "Corroded" and "Corrosive" status, it's as if they couldn't make their mind up. But I digress...

[Stainless Steel Devil Rat]

Unlike prior editions, 6e's initiative number does nothing more than ranking turns in order of first to last.  The actual number almost doesn't matter because what IS important is its value relative to the other numbers.  After initiative is rolled at the start of combat, you could ALMOST just forget what your number was as the only important thing is who do you go after... except of course for the potential of changing your initiative value post-start-of-combat.  If it wasn't for that, you COULD just forget what your number was because 6e doesn't have a phased approach to a combat round where you start on X, go to X-1, then X-2, and etc until you reach zero.  If you want to do it that way there's nothing wrong with doing so... it's an unnecessary complication but as you say if you have a computer doing it for you, then that's not a problem.

Ditto for tracking what "phase" an effect started in, in addition to which round.  If you have a computer tracking it, bully for you.  But otherwise, I still suggest and recommend just ticking off one round's worth of duration for every "by rounds" effect in play at the end of every round.

[Odsh]

Indeed there doesn't seem to a lot of actions that you have to do on your turn and that inflict short-lived but powerful effects.

I'm still wondering about the Barghest's Paralyzing Howl though. The duration is "Special" and thus should be explained in the description, but it isn't. If the duration is short, the difference between a Barghest playing first or last in the round could make a huge difference if you don't "track the phases".

[Xenon]

I suggest that you let the Immobilized Effect and the Paralyzed Effect to last Magic Rating + Net Hits Combat Rounds.

[Stainless Steel Devil Rat]

In the case of the Barghest's howl:  I'd go with "it lasts until it releases you (fat chance), until it is killed, or until it escapes from combat."  Of course, in practice that just ends up being "until the end of combat".

Why do I look at it that way?

5e inertia.  In that edition this would have been a Sustained effect. And that means, just like a sustained spell, it lasts until the creator of that effect quits concentrating on it.  A difference in that edition between sustained spells and sustained powers was that the latter did not incur a -2 dice penalty to all other actions.  So, in effect, the Barghest could lock you down until it chose to let you go, or "quit concentrating" due to being dead or being more worried about hiding after having escaped from a hoop-kicking from its victim's chummers.  I see no reason it can't or shouldn't continue to work that way. 

But it doesn't have to.  It's, as I said, literally GM discretion   Certain statuses might be full-on permanent, persisting even beyond the death of the critter that imposed them. In rare and special contexts.

I should have re-read the rules for duration (pg 221). Yeah, "Special" means it's supposed to have a specific duration called out and Paralyzing Howl/Touch do not.  Blarg.  Call it sustained.  Do it on Magic+net hits as Xenon suggests.  Figure something else out entirely.  It's a hole.  Thanks for pointing it out.

[Xenon]

When in doubt, double check how it was resolved in previous edition. Odds are the mechanics are still the same.

[Odsh]

Works like a charm by the way.

[MercilessMing]

Hey you made some cards!  I did that too (but physical), thought it would be a fun prop at the table.  I ended up combining a few just so I didn't have so many cards though.  There are a few too many statuses.  Did we need both Dazed and Confused?  Hobbled and Fatigued?  Frightened and Panicked?

[Odsh]

Well if I'm not mistaken, some statuses are not used at all (yet I assume). Like "Hazed" that prevents you from astrally projecting. I guess they paved the way for future extensions? Or are they suggestions for custom hazards or NPC powers to be defined by the GM?

[Stainless Steel Devil Rat]

I'd say: both.