I think it depends on the subject, for the sciences I started to revise about a month before (would have been later but get more nervous in science exams than any other subject for some reason), for History I'm doing it as we go along, and making sure I get everything. I've never tried cramming, as I'm afraid of the consequences due my unreliable memory (remember loads, forget important bits occasionally).
Depends on you, but for tricky subjects/exam techniques then a month before at the least imo.