Tom pretty much hit all the points on this one.
There is no set schedule at all, and it varies somewhat by division. There are occasionally field promotions, and sometimes there are the big lists; it depends, but we try to take note of all active members to the best of our ability. Typically the lists are every couple months (or for special occasions, like Christmas or the eGO 3rd Birthday list), but it all varies a little.
And of course, never ask for a promotion, but that's common knowledge.
I'll close this to prevent spam, thanks for answering Tom!