Too be honest, I don't think you can work in 'hours per day'. Because, in all seriousness (this isn't something I'm proud of believe me!), it takes me about, literally, one hour to revise properly just one page of my revision guide. And lets say roughly there are 100 pages to a revision guide, and I have 10 subjects. That would mean 1,000 hours of revision would guarentee me exam success! I know that I work too slow, but I hate rushing revision because I only forget it the next day, and have to do it all over again.
Instead of thinking in terms of time, think in terms of topics. ie. don't say I'll do 3 hours of revision today, say: I'll try and cover homeostasis and hormones today. Works better in my opinion. But, good luck, whatever. The best thing is getting started - as long as you're started you'll probably be fine!