I'm aiming for 2/3 hours, but i wouldn't kid myself and say 5 hours or more . . . lets face it 80% of that would turn into procrastination. Anyway, the amount of revision depends on your subjects, 2 of mine are languages so they don't require much and the other 2 (maths and economics) require some more. It also depends on how much you need revision - if you are genuinely good at your subjects and enjoy them chances are you won't need to revise as much as someone who lacks ability and is just doing them to impress.