im in the same boat as you but i think imma do 6-7 hours on weekends in mid april to mid may, but now, it's just making notes and making sure i understand the specfication. on weekdays, ill do all my h/w first then start any revision. i believe the above will work coz i did it for my GCSE's. good luck mate
It depends completely on how capable you are academically, how many/what exams you are taking, and how determined you are to get the 90%s. I'd suggest you do a few past papers to see what you don't know and where your weaknesses lie to see how much revision you need to do, then factor that into the amount of free time you have.
Supposedly on average you're meant to revise 5 hours outside of school per subject per week. Totally dependent on yourself. Don't stress yourself out with a massive workload though, only do the amount you're capable of, it drains you.. for quite a while longer than you think.