The current policy is that site self-evaluations are now on a 180 day schedule (so, basically 6 months). We had discussed some measure of staggering but I do not believe that was implemented.
The process of graduation involves a queue. Once we identify a site that we find to sustain its growth in an appreciable manner, we add that site to the queue. Then, the design team gets to crack at the queue and come up with a site design, which is a fairly lengthy process. We don't perform the graduation process until the design is completed, which thus means that at no given time can we give an estimated time that graduation will happen. For example, back in March I announced to the Japanese site that they were in fact on the road to graduation, so the time it takes is pretty visible.
On average, there's not a huge announcement factor associated with evaluations. We generally only bring up things if we detect something terribly wrong going on with the site. Otherwise, anywhere in the spectrum of possible evaluation interpretations from "Barely passable" to "Flying colors" all produce the same result, which is to say absolutely nothing immediate. We won't keep it a secret if a site is in our graduation queue, but we see no reason to hype it up with a big announcement when we can't even guarantee when it'll happen. To that end, unless we are asked we remain silent about being in the queue until the design team starts posting preliminary designs. This then results that the reviews themselves mostly produce no announcements or any serious news in most cases.
After some thought on it, we started to feel that 3 months was too often. Reviews are a valuable exercise in judging the site health. We want folks to put time and effort into these, and doing them too frequently seemed to dampen the enthusiasm to folks. It even felt like people found it as some kind of looming standard to worry about, rather than a simple heartbeat check. So we're looking at every six - it gives sites more time to evolve and grow up on their own without feeling like another review is just around the corner.