Personally I would aim to do a minimum of 2.5 hours work a night from September (homework and revision), with 4 hours over the weekend.
When it comes to the month before your mocks, I would increase this to 3 hours a night and 8 hours over the weekend. Try to maintain this amount if you can from January.
By the time you get to the month before your actual exams, you should be really aiming to do as much revision as possible. 4 hours on a school night and 9 hours on weekends/holidays/study leave is a good target.
Good luck!