This blog post addresses your question. Basically we stay in beta until we are deemed "viable."
Our ratings as of 9/3/12:
4.1 questions per day
Needs Work – 15 questions per day on average is a healthy beta, 5 questions or fewer per day needs some work. A healthy site generates lots of good content to make sure users keep coming back.
Excellent – 90% answered is a healthy beta, 80% answered needs some work. In the beta it's especially important that when new visitors ask questions they usually get a good answer.
195 avid users; 1,942 total users
Excellent – Every site needs a solid group of core users to assist in moderating the site. We recommend:
150 users with 200+ rep (currently 195 users with 200+ rep)
10 users with 2,000+ rep (currently 16 users with 2,000+ rep)
5 users with 3,000+ rep (currently 7 users with 3,000+ rep)
1.7 answer ratio
Okay – 2.5 answers per question is good, only 1 answer per question needs some work. In a healthy site, questions receive multiple answers and the best answer is voted to the top.
Excellent – 1,500 visits per day is good, 500 visits per day needs some work. A great site benefits people outside the community. Eventually, 90% of a site's traffic should come from search engines.
A number of betas have been waiting longer than we have!