As long as your greek advisor is behind you, and you are close/can be at school everyday, you could probably start contacting the NPC as soon as you want to. The process takes awhile, and your local college panhellenic has to grant you the right to expand(this became a HUGE sticking point on my campus).
Once the local panhel grants the right to expand, letters of notice to expand go out, surveys by national get filled out, nationals get picked for the next round, presentations get made, i think i'm skipping a few steps, and then the sorority gets chosen. It usually takes about a semester, but since we did most of the work over winterbreak- we got voted on before thanksgiving, and then by the martin luther king jr day, we were on campus.
Just think what are the most important aspects of your *local* sisterhood, and stick to those; make sure that your national is willing to work with your group. our biggest point was we wanted to keep our local history, and our local traditions...and tri delta seemed to be one of the only ones willing to work with us-they were the only national that knew our local's name, and used it often. this impressed us (turns out there was a reason for this- if your a tri delt, you know what i mean
)
Basic Idea: Stick to your guns...make sure you have ALOT of time devoted to this...become best friends with your greek advisor...and GOOD LUCK!!!!!