I recommend to remove the "beta" tag from "Quantitative Finance beta".
"Beta" makes the site less sincere and professional. The "beta" version remained for some years now, I dont see why it should be called beta anymore.
We actually don't decide when to leave Beta.
Our site's stats are tracked on Area51 and you can see them here.
I'd say we have a good number of visits, but as you see our percentage of answers needs work as well as the number of questions per day, which is closer to the "Needs work" grade than the "Excellent" grade.
Everything SRKX said is accurate, but while we're at it, I want to clarify that Area51 stats are not the only criteria Stack Exchange uses to determine whether a site graduates.
What else is there? For one thing, does the community engage in self-governance, hashing out site policies on meta? Is there plenty of voting and editing? What is user retention like? These are indicators of site health, and we take them into account too.
This is because our goal is to create an archive of knowledge about quantitative finance -- a place on the internet filled with canonical information about this subject. When you post here, you are helping a future person who is struggling with the same question you struggled with. It is a collaborative endeavor, and to achieve, we've found it requires thinking about the humans behind the posts, as well as the numbers.
If you're looking to graduate, its wise to keep an eye on Area51 stats, but don't don't stop there.
A quick update to let you know that Quant has met the criteria for graduation (according to a note by the Stack Exchange Community Team).
In other words, we are going to leave Beta soon! The exact graduation date is not yet known though.
Anyway, congratulations and thank you for all your hard work and dedication building the QE community!