Lame_Wolf - 3:25 pm on May 9, 2012 (gmt 0)
I've heard that too. But i've also heard of people with less than that, and became premium publishers.
...but I seem to remember it in terms of "millions of pageviews per month" that I once heard from someone who was a premium publisher.
However, I cannot see a Google employee writing to someone to let them know that they are getting close to being a premium publisher.