GMAT + MBA
The ideal time to start planning for your MBA or EMBA is 1 year before you intend to apply. For example, if you want to apply in September 2015 to start in Fall 2016, you should start now. Beginning early gives you enough time to prepare for and ace your admissions tests, take more responsibility at work to improve your profile, be nice to the people you want to write your recommendations, and not lose your mind as you try to work and apply.
For MBA applicants, you should start early because your best chances of acceptance are in Round 1, the September to December application period. EMBA students should keep in mind that most programs have multiple intakes (September and January) and rolling deadlines. However, applying early still gives you the best chances of acceptance.