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.
This seems like the worst of all possible worlds to me, Hobbes. Like, here are possible ways it could work, given your example (B uses Full defense on A's turn):
* lasts until next turn - B is protected only against A's attack. They will have to use Full Defense again against C (and won't have a Major Action for it unless they have at least 4 minors). Its like they are getting "half" value for the action.
* lasts until the start of your turn on the next round (I think this is what SSDR was suggesting) - B is protected against A's attack, C's attack, and ALSO A's attack on the next round. Its like they are getting "one and a half" value for the action.
* lasts until the end of the current round (or start of next round) - B is protected against both A and C's attack on this round. This seems like "full" value for the action, to me.
(now, if actions refreshed for each person on their TURN, not at the start of each round, I think what you are saying would make perfect sense. That would be an equally valid way to set up the system, I guess, that's essentially the way D&D 5E works, for example.)