Getting the best grades is less about how much you study, but more about how effectively you study.
The key to A-Levels is past papers and the specification. Do all the past papers, even if you haven't learnt everything for it yet, because you get used to random questions (Like a biology one about a seal), and many questions reappear.
Use the specification as a guide to what you need to cover, rather than relying on one of the many dodgy textbooks. You don't need the random stats they give you. I've broken my specifications down, so including all the info I need below each spec point. This has enabled me to finish one of my courses before we even started year 13, so I can focus on my other subjects. (This is probably easier for science then it is for humanities. I did history at as, but getting a D in every module, I don't really feel qualified to talk on this)
In terms of time, you need to study a bit, but I wouldn't say study each day. Certainly each week, but if you're doing high-density studying, it doesn't need to be everyday.