I believe there are 3 stages:
About 9-12 months before the exam: This is for the ones who can afford a long, hard preparation - say you are not good at one or two areas e.g. in quantitative (for non-science students) or verbal (for engineering students). You get enough time to build these skills and get disciplined. Its also suited for those who have been away from academics for some time - 2+ years.
About 3 months before: This is kind of what I think most test takers do. Spend about 3 months working on your skills, getting in the flow or the rigorous exam - not too long but just right.
About 45 days before: This is more of the 11th hour thing, works well for practicing test taking, understanding strategies and other test taking tips. Not good if you want to clear your concepts. Good option for busy professionals.