Fairly straight forward question. Adamantine Mind says that if the caster fails their will saving throw in response to my successful save, they're stunned for one round. Since the stun happens during their turn, does that mean they're stunned until the end of their next turn or the beginning of their next turn?
I don't know Adamantine Mind but I guess it goes the same for all effects/spells duration while in combat.
The effect should last until that same moment the next time that character plays. If the caster was able to cast, his spell resolved so his round was mostly finished and then Adamantine kicked in, I'd say that he is stunned until the end of his next turn. (therefore, he is only prevented from playing a single turn).
I guess you could push further by saying that if the casting was his first (standard) action, then you could rule that he is stunned until midway in his next round, allowing him only to take a move action on the next round since he was stunned for the first part of his round, which lasted enough time to prevent from making a standard action.