I usually try to do like about 4 or 5 hours a day, but I spend a lot of that time not really studying, because I suck at revising. I usually study for about 30 minutes then 'break' for about that long as well.
I'm like 2 hours revising - 30 minute break. Though I have no lessons on a thursday or friday, so I spent all 7 hours revising then, with my 15 minute and 45 minute breaks.
I don't really take breaks, I know it sounds crazy but its what works for me. The breaks I have are usually dinner or standing up and looking outside from my window to get some fresh air.
I think that you have to find a way that suits you the best because everyone is wired up differently.
I revise about 7 hours straight (on average), only toilet breaks every hour or so. I can get a lot of work done this way and then look forward to the evening where I can just chill
I do about 4 hours per day, but thats because i started revising really early so that i learn everything and dont really need to cram it all in a week before exam time!