Agree with all the sentiments here. I'm coming up to the 40hr mark and *should* pass on the scheduled 45 hours but probably won't because of the stop start weekend nature of my flying. I only have a target in my head (~50) as something to aim for - i.e. a schedule of what I want to achieve in the next 10 hours.
However another way to look at the question is... If I'm at 'x' hours and not making progress then is it time to stop? One of my previous instructors talked about another student. He was on to 35 hrs and still hadn't mastered the flare. The instructor had a sit down with him and said that perhaps flying wasn't for him and that he didn't want to take any further money from him as it was wasted.
I guess therefore that there must be a limit whereby if you aren't ready for QXC or the skills test then it is time to reconsider.