[here be 99% accurate crystal ball predictions]
I'd venture a guess that we're close to leaving beta. Tex.SE currently gets two Excellent scores on area51 (on number of users and percentage answered questions) -- and by a large margin. Two stats are close behind (number of questions and visits/day). Of those, the visits/day stat has been climbing steadily for the last few weeks. We used to have excellent rating in "average answers per question", but fell behind by a small bit. FWIW, this last stat is probably not very applicable to TeX.SE, where there's usually one preferred way to skin a particular cat (unlike, say, programmers.SE or gaming.SE).
So, I'm fairly convinced we're going to launch in a few weeks. (At least, I hope so!)
Update:
The Powers That Be said that
There’s no harm in staying in public beta far beyond the initial 90 days, so long as the quality of the Q&A is high and it’s not a ghost town.
We have extraordinary signal-to-noise ratio (in a highly-scientifique experiment, I counted only 55 or so questions of zero or less score (some of those are just too obscure to get upvoted), and answers are rarely voted down -- because they are relevant. TeX.SE is not attractive to spammers, promoters and vandals -- seriously, what can you sell to {Con|La|Lua}TeX{t} users?