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?
The best way of revising would be flash cards and revision time I would spend 2 hours reving with 10 min breaks every half an hour to let your brain take everything in so your not overloading your brain or going off topic this will help bring your grades it, I would also have a bottle/glass of water and a sweet snack as it keeps you going until your break. My advise would be don’t panic too much as that’s when it stresses you out just stay calm and try your hardest because that’s all you can really do