I started doing about 7 hours a day from 11am to 6pm in March, with a timetable for all subjects, but now I'm doing about 8 hours a day from 10am to 7pm, with a couple of small breaks in between, for whatever exam I am prioritizing at that time. I find it easier to revise for longer with less breaks, as I just get distracted and my revision doesn't flow as well as it would otherwise. At the moment I'm not using my revision timetable anymore, and I'm just doing what I know I need to revise, for example I'm going to spend Saturday doing Unit 2 Psychology, and Sunday doing Cold War.