It is based on the number of posts you make, and changes witht he post count. Please don't take it apon yourself to gradute at a fast pace (if the thought had crossed your mind). It has been argued post counts shouldn't even be shown as it does created useless posts by users whom just want to have the highest count, but has yet to be removed..
Anyways, im not implying anything towards yourself, just making a simple statment. And again, just to asnwer your question, it is based on post count.
