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.