Never too late to start revising. I don't know about you, but when I did it, I'd worked pretty hard in school so I already pretty much knew everything, and as a result didn't really revise that much and felt guilty about it. I was going over material ridiculously quickly the night before most exams. It actually suited me just fine - I did really well in them. I'd say if you manage your time well, a week is more than adequate