I know we just got started,but what is the criteria of getting the site complete the beta and become part of stackexchange? just curious!
A site will stay in Private Beta a minimum of 7 days. Typically, in my experience, most sites go to public beta right on schedule, although some do require additional time in private beta, due perhaps to low traffic or other metrics not meeting standards. And some never leave private beta, and are closed before ever entering public beta.
Once out of private beta, there's a minimum of 90 days in public beta. Although here, experience shows that many sites stay in public beta for a very long time. Some well over a year.