How early should I start preparing?
It is suggested that you start one year from the deadline because it is a long process that involves multiple steps: taking the GMAT, a language test (for non-native English speakers), researching and school visits, choosing your recommenders, and writing your essays.
Consider the fact that you may have to take the GMAT twice to get a better score. Starting a year early gives you ample time to go through all these steps without feeling rushed.