6 hours a day is a little harsh on yourself, my maths teacher advises 3 hours a week for maths and 3 hours a day combined for all our subjects, in the end I did not start revising till well into study leave for AS and A2 and came out with 2A's and 3B's at A2, for AS my grades were ABBBB with only 2 days of 6 hours revision for each subject.
So in all based on my experience in taking the same subjects as you and GS you do not need to revise excessively, but it depends on you and where you are looking to go to uni, as well as this i would advise to go out and get so experience you can put down on your personal statement when you come to write it, as a well written personal statement showing you are a well-rounded individual is just as useful as predicted grades when it comes to uni applications, as everyone now can boast high grades e.g. 27% of people getting A*or A grades this year.