- Number of members?
- Number of questions? (total or per-day)
- Number of answers?
- Number of visitors and views?
- Complete a period of time? (six months? one year?)
A necessary condition: at least 10 questions per day mean the site can be considered for graduation. Graduation, site closure, and a clearer outlook on the health of SE sites
But even if a red-hot beta gets 10 questions per day for the first several months of its existence, that doesn't automatically make it a graduation candidate. It takes a long time for a community to become large and engaged enough to support moderator elections in which more than a handful of people show up.
Excluding special projects, the youngest site to graduate is Network Engineering, which started in May 2013 and graduated in June 2015. So, I suggest revisiting this question in the middle of 2017. And chances are that by then, the graduation policies and procedures will be different.