I'm in the exact same boat.
A couple of months left before exams.
I've asked a couple of my teachers about this; Biology teacher said to start off slow, a couple of hours a week. Chem said the same.
Start increasing time revising as exams get closer; especially when Easter comes around. That's when it gets serious.
For now, over the week - 3 to 4 hours per subject is more than enough; apparently, any more than that and you're overworking yourself.
Goodluck!