My chapter was a local sorority for 2 years before we went national. The group was initially formed with the intention of becoming a chapter of AEPhi, but the campus Panhel didn't want to extend to another national at that time, so a local sorority was formed instead. 2 years later, we approached Panhel again, and they agreed to let us seek out a national org. Since we'd been interested in AEPhi from the first, we said so in our letter to NPC - AEPhi came to present to us, and a match was made.
There's no real set time limit, unless your local IFC or Panhel sets one. I don't know how fraternity colonization works, but if your local is strong on campus, go ahead and seek out nationals that aren't already on your campus. The worst they can say is no - but somebody's bound to say yes.
Good luck