Personally, I'm Year 13 and I do 4-8 per day, depending. E.g. I'm about to finish my 8 for today, but knowing that I'm going to a uni open day tomorrow meaning that I'm unlikely to do more than an hour or two. I won't often do 8, either. On the other hand, I have Church Sunday mornings and prefer to have a relaxed day, so most Sundays I won't do more than 4. As a year 12, you certainly don't need to be doing 6/7/8 hours! Perhaps soon before your exams 6, but your priority is long-term revision motivation and ensuring that you understand all your topics. If you did 4 hours each non-school day, you should more than cover the spec in time, and you'd have time to relax as well - basically, do what you can but don't force yourself to do more than you can manage. Also, you shouldn't give up days out or anything at this point - by spring next year you probably won't be socialising much so take the opportunity now!