Next time you take 30 minutes, she might just go on home and ignore you the rest of the night, because that's what high schoolers do. Instead of playing a game back, you need to communicate to her why you need to take the time and make her understand, not have a negative attitude back and cause more games to be played.
But honestly, there's a lot of drama to be had with "being a pushover" taking extra time in the shower really isn't one of those things, if it is, you two are probably playing a shitton of games with each other.

You can "not be under her thumb" by just communicating with her, if she chooses to have an attitude or not compromise with you, that's her own problem. But playing games back is high school s**t.