What if you don't count sessions? I have no idea how often we reset. Last campaign reached into the epics and went on for a couple years. What is wrong with reseting if anything? Once the story is told it's told, sooner or later you reach a point where you have to move on right?
My university gaming group has had three "campaigns"
1) Oriential Adventures went from September to December (every 2 weeks). Total sessions 7
2) D20 Modern Zombie went from October to December (every 2 weeks). Total sessions 5
3) Generic "Greyhawk" campaign started in January and is still going, Sessons so far: 15
I find the format we currently use, a generic "D&D fantasy" setting with rotating GMs every 4-5 sessions, works really well. We don't see any end in sight and can probably go on for a year or so.
That's a difficult one to answer, since I haven't kept track all that much. I think our average is about 40 sessions or so; for many many years we reset every 1-2 years (for weekly play, missing maybe about four sessions a year), so for some campaigns the reset has been around 90 or so sessions. For some, the planned length of the campaign is shorter so we'd reset in say 20 sessions or so.
I don't know if my groups have a "usual." I've both run and played in campaigns that lasted over 30 sessions. I've both run and played in campaigns that lasted under 10. Sometimes, things just fall apart, but other times it's because the campaign has reached its conclusion. (Some of my campaigns are one long story, and thus have a definitive end. Others don't begin that way, but eventually become a single over-arching plot, even if there are some games that don't tie in to the main plot directly. What can I say, it's the sort of game I prefer as both a player and a DM. )
60 now, and taking a break for our first child. I was thinking about a reset, but mysteries are about to be uncovered once again. And I am considering using a class-less, level-less system to allow more options in critter and character creation, but that takes work.
I guess we throw the curve off a bit. We started in 1982 and have had 924 sessions with our original campaign (plus another 60 sessions with a second campaign that ended a while back).