Just saying as it is in my case,
I got 5 A*s at A Level. including Maths Further Maths, Chem Phys, General Studies
I was pretty much a workaholic during most of Year 13, particularly April of Year 13 onwards! A typical day for me was getting in 9 hours a day.
BUT I discourage this sort of approach in hindsight,I achieved what I set out for, including unconditional offer at Cambridge for Phys Natural Sciences, but I think it is fair to say I could probably have still done this, but been more efficient with timing and what I learnt etc. and saved a lot of aggro. To explain what I mean, because of the extra work I would do in my sciences, I did not find the infamous interviews too bad at all and believe I did pretty well. However many say they found theirs horreddous. I feel the extra stuff beyound the A2 syllabi stood me in good stead for Year in Industry and Cambridge interviews.
So I guess, it's what you want if you simply wish to meet a conditional offer for a competitive university course, then I wouldn't be too religious about it. But if you want to pursue extra things for academic interest in its own right (which I feel hold you in good stead for the demaning university courses), then I believe your revision should become more like a full time job!!