Latch there's been a lot of talk about that in the other communities, and no one can pin it down to any one thing. It's not necessarily usage, because I'm pushing 100-300MB through that mailbox every day. It's not necessarily a matter of time because I got my first three invites within 24 hours, and then didn't get any at all for over a week.
My personal thoughts, every time the Gmail system reaches an internally guaged milestone and remains stable, they figure out how many users will be needed to make the next milestone, and send invites accordingly.
I also believe that the longer you've been in the beta, the more invites you get. I say believe because I really have no idea, but damn if it doesn't make sense to think that way...
