As can be seen from the Area 51 page, there are a lot of sites over 90 days old, in fact, the oldest are over 1000 days. That said, how do we stand so far?
First of all, I'm not a SE employee. That said, I'd say that this site is so far on a good path to sticking around for a while, I don't see it being closed anytime soon. We do need to keep up questions and answers, and invite more people here, etc, but we aren't in any immediate danger. Furthermore, there is a healthy community developing here, and our statistics except for page views look pretty good. Page views are the least important statistic for determining if a site will remain opened, so we should be good for now.
That being said, I don't think we are going to leave public beta anytime soon. Most sites don't leave public beta for an ever increasing amount of time, and as mentioned, there are sites with 3 years in public beta. I've asked the question as to if sites are staying in public beta too long, and Jeff gave a pretty decent answer. The bottom line is, they look for a kind of runaway growth in the site before they will take a site out of beta. And even at that, when the decision is made to leave beta, it seems that it takes several months at least until the site can be designed. Graphic Design was promised on August 6 that they were on the short list to launch fully, but they still haven't. Thus, even if Stack Exchange decides we are ready at 90 days, it'll probably be at least 3 months before we would graduate and become a full site, maybe more, just due to resources.
Stick around though, it's a good group;-)