A month and a half is alot of time, if you use it wisely. My revision is slowly increasing, and when the easter holiday hits - it's just past papers for me. So no, why would it be late?
I don't do any similar subjects to you but I do past papers, seems to work well with mathy subjects. As for the time limit, It all depends on the person, I just revise until I understand the content or it's engraved in my brain etc. but I usually spend 3ish hours..but thats me.
Good Luck!