You don't have to do much revision. I know you all think they're really hard. I did when I was in year 11, but GCSEs are just a test of technique; heck, I didn't understand much chemistry, it's just that the same stuff comes up year on year and so revising from past papers is the best way, not wishing to boast, but with their technique, I would be finished well ahead of time in all exams, and in Chemistry I got an A* with a UMS of 199, so not bad overall...
I was recently reading something that said GCSEs should be revised for 3-4 hours a day, A Levels 4-6 THE ONLY EXAMS THAT REQUIRE 8-9 HOURS OF ADDITIONAL STUDY ARE YOUR UNIVERSITY FINALS.