It varies per subject, but I'd say no more than 4 hours a day, as long as it is good solid revision! Put more into the subjects you're not so good obviously, but the best thing to do is to see how much you have to revise for each topic and how long you have until exams. Aim to get it all done by a certain point (at least a week before the exam so you can go over it again) and then work out how many hours a day you need to do from that.
Put it this way, I'm now at A2 and minimum is 4 hours a day according to teachers, and I never do more than 5. Your brain has to relax as well remember!