We're every two weeks in theory, but in reality, probably every three weeks. We had some downtime late last year that brings our average down a bit too.In both games I'm in, it's often "Every two weeks", but sometimes you'll have three weeks between sessions.
Urk... I didn't realize it was specific to D&D. Still, I get multiple 'games' in a little more than twice a month.
Why? My group meets regularly over Maptool every Tuesday morning (well, my time, we're on two different continents) every week for three hours and has done so for the past four or five years. My Sunday group meets 10 pm Sundays (again, my time, this group is on three different continents, so I have NO idea when they are) and plays for a bit more than three hours usually.
Or are you talking about something like DDO or other MMO's?