She giving herself two hours simply because she has something else to do during this day. I must admit that if he can't reverb her back in less that two hours they risk to have a problem in the future. They will have class and my guess is that this risk to happen a lot. They can't afford to miss too much classes. Peter need to be able to turn her back to human in ten to fifteen minutes like the previous time. Of course, I doubt that Sarah will mind if Peter continue a little more after she's back to human. Afterall, it's Peter only true skill.
Ce qui est un problème pour l'avenir du couple, il faut plus que ça.
S.P.P.