That depends on your ability.
Some people can achieve B's - A*'s at GCSE with no revision whatsoever.
Others can't achieve 5 C's or above with more than four hours of revision per day.
Personally, I think that four hours a day is too much, nevermind a full school day. On the other hand, if revising that much is going to improve someones grades then I'm certainly not going to discourage them from revising that much.
For the best advice, I would suggest asking each of your subjects teachers about how much preperation is needed in that subject for you to achieve your target grade (or the grade that you want, as long as you're realistic).