Setting a rigid amount of time for revising is pretty pointless if you're not revising effectively. You've got to find ways that suit you and actually allow you to absorb the information you're trying to learn - for some this involves re-writing notes many times, making posters, making flash cards and then looking through them periodically. try different things and see what suits you best, and then see how you're improving. Once you know what works best you'll have a much better idea of not just how long you need to spend revising, but what exactly it is that you need to revise.
One of the generally accepted best ways to test your knowledge and find out what you need to revise is using past papers. Do as many as you can for each subject (although try not to exhaust all of them too far before your actual exams), see where your weaknesses are, and apply the revision techniques that you find help you best.