From Graduation, site closure, and a clearer outlook on the health of SE sites:
When a site starts to consistently receive 10 questions/day, we’ll consider it for graduation.
While this isn't always true, most sites don't end up graduating before reaching ten questions per day. The other Area 51 statistics play a much smaller role in determining if a site is ready for graduation.
Other than ten questions per day, everything else is site-specific. Once it's clear that a community will still work well with the increased reputation requirements and other effects of graduation, it's likely to graduate.
What targets should we as a community focus on in particular?
Bringing in more users and encouraging them to participate. A larger and more active community means a better site, and this will also bring us closer to graduation.
Also remember that it is not a race to finish beta. There are plenty of useful, healthy sites that have been in beta for 5, sometimes almost 10 years. They still produce good content, but aren't quite ready to graduate. It's okay if we stay in beta for a long time; as long as this community continues to grow and OR.SE continues to produce helpful content, we'll be making the world a better place and moving closer and closer to graduation.