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.
Is your site in between these two categories? You don’t have anything to worry about. Regardless of how small the site might be, you have a home here in the SE network.