I actually think this is pretty earlier to start revising - for AS anyway. You have a lot of time left, more than you'll need I think. I started revising two weeks before exams for Biology and English lit, and about two days for Maths and Physics, and I had As in all but one of my modules. I must admit though, I was pretty lucky!
I think the best advice is not to panic, and start working on your weakest subject first. If maths comes naturally to you, I don't think you'll need anything more than a week to prepare, so I would concentrate on essay based subjects first. Oh, and do lots of past papers!