I guess it depends on how strong you are in a subject and also on other stuff like your schedule and how much you prefer studying on a daily basis. For getting good grades I would suggest about 5-6 hours. I've been through it myself. In fact I finished my GCSE course this Jan. If you're comfortable with a subject don't bother spending too much time for it. Spend a predominant portion in something you're unsure of. Maybe 1hr extra for a subject you're doubtful in. Hope this helps. When are your exams anyway?