I usually drop tank stance when party is all okay with it and after 2 enmity combo rotation assuming my gear is at level or better. Or if no one says anything, then I just keep tank stance on until I'm 100% confident I don't need tank stance any more, which usually depends on whether the party used any aggro mitigation. If I don't feel comfortable dropping tank stance, then I don't (except for EX content where tank swaps are required, lol).