Campaigns vary wildly. Let's see, my longest campaign will hit 17 years next spring. We went from playing weekly (or more often, ah, being a student again) to meeting three to four times a year due to all of us having moved away, having families and jobs and whatnot. All in all, there were close to 20 players involved, some for just a session, some for more, with the core group being 8. It's a Vampire campaign that started in the Middle Ages and is now in the 1990s. The characters are crazy powerful, but since they deal with even more powerful entities all the time, it is still OK - although the rules get really strange at these powerlevels, and we mostly talk a lot. The centre-piece of the campaign is the players' library, now close to 200 texts, all handouts (and recently digitalized and indexed on my RPG-Wiki), which holds hundreds of sessions worth of lore together. This campaign is the shining jewel of my RPG life, with fantastic players keeping it alive even during difficult times.

But I also had campaigns fizzle out after one or two sessions for varying reasons. The last campaign we finished was 15 sessions (Ravenloft with Pathfinder rules). A campaign that is on hiatus now due to me moving away is at 60 sessions in 5 years, and could be finished in ~10 sessions, I think, but I do not know if we will manage that, although it certainly deserves a satisfying ending (Midnight with Pathfinder rules). I have a slow Spelljammer with Fate rules campaign around 25 sessions, a great The One Ring campaign at 15 and a very promising Edge of the Empire at 5.

Before I moved I held a regular Open Gaming Table once a month, usually playing a non-serious Megadungeon, but often doing one-shots trying out new systems and/or settings / adventures. I simply sent invitations to the ~50 players on my mailing list and the first handful to respond got a slot for that session.

In conclusion, campaigns vary from 1 session to many hundreds spanning 17 years. I feel it is a lot of work to keep a campaign alive, not only for the GM but also for the players. There needs to be a certain chemistry that is hard to predict. I lost campaigns I had high hopes for and kept ones that were only meant as funny one-offs.

My longest campaign was a game I ran 1-2 times a week for about two years in college, but it failed due to attrition (a few dropouts, I went overseas for a year, etc).

My recent campaigns have been ending due to moving for work (need to go where the money is).

For EotE, my old group was reaching the 300XP range and was barely even scratching the surface of their characters and only touching upon the surface of the story. I've also had players build characters with 300-400XP just to see how broken it would get, and it resulted in specialized (but not perfect) characters that still had room to improve.

Really, it's however long the story can last and the game mechanic can handle the necessary gameplay to match the time.

My longest campaign ran just over 10 years, which translate to about 100 episodes, broken into 4 "seasons" (with most of the seasons broken up, cable TV style, into two "half seasons").

I've been RPGing for about 35 years, acting as the GM more often than not. In that time. Since around 1990, I've played with iterations of the same group. New folks join, many folks leave, but I can trace the "lineage" of the group back to the summer after my sophomore year of college. Up until around the turn of the century, we often played weekly (or sometimes more than once a week, if we had multiple games going), but real life has reduced that to a roughly monthly occurrence now.

My 10-year Star Wars campaign (which was, itself, a sequel to a short-lived campaign in 1999 that lasted maybe 8 sessions) is definitely my longest chronologically, but may not have been the most sessions. I did the math once, and I think I've GM'd something like 20 campaigns, about 10 of which have lasted a year or more. Some were created to be open-ended, while others were supposed to last for a limited run. Some had great potential, but fell short. I think my longest after Star Wars lasted 3-5 years each (an Avengers-type superhero game, a "super" game where the premise was "What would heroes be like in the real word?", and an Indiana Jones-type game where the players worked for the Vatican). My biggest disappointment was an X-Files inspired campaign that fizzled after 4 story arcs (6-8 sessions).

We're rolling up on "episode 3" of my Firefly campaign (using the EotE ruleset) and I'm hoping to keep it going for quite some time!

I'm not sure. This is my first kick at the can and we are a year and a half in. Granted, our adult professional lives limit us to playing once every month but I project this campaign to last at least to the two year mark before we grow tired. There are story plans into a third year, but who knows if we will want to continue at that point.

Not long enough.

My one and only campaign EVER ended when all the players (other then me!) went off to university. We're supposed to play when they're on breaks but that's already falling apart with the first break on its way.

First game is going on 6 months now, running every week. Just started my second game which runs every other week, and this will be the second session coming up.