We were advised that we should spend approx. 10hrs per week on each subject, including lesson time e.g. 5 hrs lessons, 5hrs self study, but I found it was rarely necessary to do that much - I did history, geography, philosophy and biology.
Probably did do approaching that amount for history and philosophy at A2 but that includes extra reading around the subject. More like 2hrs each a week at AS.
In contrast Biology is just straight learning so very minimal - probably often less than an hour a week if you focus in lessons
Geography fluctuated depending on when essays/questions/projects set especially as split between 3 teachers, but probably averaged 2hrs a week.
It will vary greatly depending on the individual though - I got 5As at AS and hope for at least A*A*AA next week on this whereas I know others who worked for hours and hours and would be delighted with Bs
I worked in lunchtimes/ frees and occasionally evenings - tried not to lave any til weekend as I worked all day Sat and aimed for a day off Sunday.