Well I start revising at about 8:30am and go straight through until about 5:30pm. I stop for an hour for lunch, and then I usually have small 15 minute breaks every couple of hours. It probably works out at about 7 hours a day. If you don’t think you can stay motivated for this long then don’t do as long, not everyone can stay focused for that long.
However I’d say you need to be doing at least 5 hours. Think about it, when you’re on study leave you’re technically still at college because you’re supposed to be working. Study leave is a substitute for college, therefore you should be revising for about the same amount of time you’re at college, if not more.