[Banshee]

No it's an anytime action so it should say "until start of the next round" ... which is when initiative resets

Little confusing.  NPC A goes first, PC B goes second, NPC C goes third.  B can spend a Major action during A's turn, then take whatever leftover actions at their turn, and still get full defense when C acts.

I'm betting plenty of Players/GMs will end the Full Defense Benefits at the beginning of B's turn, and give C a clean shot.  Or force B to spend multiple Major Actions on Full Defense.

The whole point of it being an Anytime Major Action is so they don't have to spend multiple actions on defense.
So since actions reset at the beginning of the round they either last for the entire round (major) or for just a single instance (minor).

[Hobbes]

The whole point of it being an Anytime Major Action is so they don't have to spend multiple actions on defense.
So since actions reset at the beginning of the round they either last for the entire round (major) or for just a single instance (minor).

I am not sure that distinction is explicitly made? 

[Typhus]

To my understanding (I think I asked something similar before), the action economy resets globally at the top of the Round for all characters.  That's because if you did get attacked before your new Turn in the new Round, you could just opt to go back on Full Defense as an Anytime Action.  However, if you were never attacked before your turn, then it's a moot point.  Top of the Round makes sense in context of the Anytime Action.