I'm doing bio, chem and psych AS exams in May. Haven't actually made a revision timetable yet (probably a job for the weekend), but my school normally says about 10 hours per subject as a minimum so 30 hours total. I've got roughly 2 1/2 weeks off for Easter so let's say about 17 days for example so I'd be doing about 2 hours a day roughly including a couple of days off. Doesn't seem like a lot, but if you do it consistently it will help, but then again it's only a minimum so some days I'll probs end up doing more than that. I think it really depends on the individual circumstances tbh though