I try to do around 4 hours a day, but I've set it out so that my revision timetable runs chronological order of my exams. I just want to see how much I can finish this holiday, but I also want a break
it's not a set timed thing, if I work by the hour I get too stressed that I haven't met my aims in the hour so I work until I'm satisfied with the subject, take a break for around half an hour, then get back to work, so I guess the amount of breaks vary with the subjects I'm doing.
For example, today I was doing physics and it was just clear it wasn't going in my head today so I sat around on my laptop for an hour and then carried on, it just changes constantly haha