That depends when you hoped to start.
For full-time MBA programs I've looked at, the applications are due between October and April, with some schools' final-round deadlines coming very early. Generally, you want to apply as early as possible as the competition is assumed to be tigheter in later rounds.
A general (and idealized??) timeframe:
In the summer: GMAT! (once, twice, thrice?)
Up to August: Program research, advance prep
Sept / Oct: applications
Nov/Dec: all apps in
But look at your schools and see where their rounds are. Make yourself a nice spreadsheet so you can be organized. And maybe check out a good book. I have Montauk's How to Get Into Top MBA Programs, but I have heard others cited on this forum.
And definitely check out the FAQ sections of each school - there's always lots of good info there.