According to our Area 51 page, we entered public beta on August 22, 2016, nearly three years ago.
90 days may indeed have been the cutoff in the early days of Area 51-launched Stack Exchange sites, prior even to that post. The post opens with it because users of one small site were concerned about getting shut down at that mark. It reassures those users that graduation could take longer:
How long can a site stay in beta?
The simple answer is, it takes as long as it takes. We’ll wait. If a site needs more activity, go out and evangelize it. As long as your site shows steady progress and continues to make the Internet a better place to get expert answers to your questions, it will march on.
More recently (in 2015) there was a Meta Stack Exchange announcement with more specifics:
- When a site starts to consistently receive 10 questions/day, we’ll consider it for graduation.
- If a public beta site does not produce consistently helpful content, and lacks the caretakers needed for flags and spam to get handled and our Be Nice policy to be upheld, it will be closed.
We are not at risk of closure, so we'll definitely stick around in some form (perpetual beta is possible). According to the Area 51 stats, we get about 7.5 questions per day, so we're making progress toward graduation eligibility. Graduation includes privilege threshold adjustments, which we could survive now, but without any 10K users it's probably ideal to hold off for a while.
The sketched graph in this MSE answer visually shows "a typical growth pattern for a Stack Exchange site." Eyeballing our current site analytics, we seem to be at the halfway mark between the sketch's start of public beta and graduation.