I typically play GM-less games as one-shots and GM-ed games as 6 session campaigns. I have run and played in Torchbearer campaigns that were much longer.
For DnD I play a lot of one-shots since it’s tricky during the school year to keep a weekly commitment. During the summer, my group can usually manage a ~12 session campaign.
For Monsterhearts, it’s really variable. Somewhere between 1-6 sessions.
I mostly play PbtA games and do a fair bit of one shots, so that’s y’know, one session (or two when we inevitably run out of time and have to wrap it up later), but when we do like “full” campaigns, I think the game sort of naturally lends itself to wrapping everything up after a few months to half a year or so, playing roughly weekly. At that point, if people have stuck with their initial characters, they’re tending to get a good chunk of their advancements, are into the special late game advancements, and have probably pretty fully experienced whatever they wanted to get out of that character, so the systems tend to reach their natural end point.
I’ll also say, slightly to what @EricVulgaris said about D&D campaigns’ length, I’ve been in multiple D&D groups trying to play for that forever campaign that’ll just go on for years, and they’ve all died off prematurely within a year or less, so I’m definite a big fan of shooting for the tighter months-long story with a game that pushes the fiction faster.
I once read on Story-Games or The Forge that the most fun is had between session three and session five, (inclusive). I don’t know where the original numbers come from, but it does match with my experience.
I like to join Gauntlet Hangouts games that run for 2–4 sessions, figuring that’s the most I can commit to for an online game on a weeknight right now. I did play in one Cold Ruins of Lastlife campaign that booked in four-session slots each month and ultimately ran for something like 10–12 sessions, and it was excellent—but I probably would’ve been too skittish to sign up if I knew from the start it was going to run for so long!
For face-to-face games, my answer varies a lot more. I probably run more one-shots than anything else, and often use these to try out games I’ll likely never play again. I’ve run a few longer arcs over the years, though; just before my kid was born a couple years back, I ran about 10–13 sessions of Urban Shadows, about 6 sessions of a Dungeon World campaign, about 6 sessions of The Flux, and just a few sessions each of some hacks I made based on World of Dungeons and Fate Accelerated. Most recently, I’ve gotten about 8 or so sessions so far of one of my own games I’m playtesting, and I’m hoping my next game will be a 5–10 session arc of Silent Titans, Invisible Sun, Phoenix: Dawn Command, Mutant: Year Zero, Night’s Black Agents, or some other game that looks especially well built for multi-session play.
I don’t really do “ongoing and open-ended” games anymore, since I find that tends to just mean “fizzles out disappointingly.”
I’d say 4 sessions is my usual deal. Me and my group have a rotation (well, sorta) and we play short games of Chtuluh dark ages, Pugmire and whatever I come up with (tested some DeD 5e houserules last time, playing a bit of Into the Odd right now). We don’t set the number of sessions beforehand but, so far, the shortest game lasted for 3 sessions and the longest 6, if I remember correctly.
I like around 12 sessions of actual gameplay - with 5 people at the table that gives enough space to give primary focus to each character for 2 sessions plus bookend the opening/conclusion of the campaign. During that I’d normally aim for 2 short arcs or 3 overlapping mini arcs if I’m feeling adventurous.
For most games in the past I tended to play 6 sessions of small games and 12 sessions of big games, not including one-shots. These days due to the monthly rotation policy on the Gauntlet I play 4-session games but oftentimes it’s not enough to go through all the planned content and finish all the character’s arcs.
At our game night we alternate between one-shots open to all-comers, and longer campaigns for our core group of 4-6 players. We’ve done a handful of 3-5 session runs, but the most memorable have been the longer campaigns we’ve played to completion, each of which (a fantasy PbtA hack, the Dracula Dossier using a home brew PbtA system, and Blades in the Dark) lasted 30+ sessions. In my experience there’s no substitute for the epic scale, character development, and depth of a long-term campaign, so even though we mix it up and have fun week to week, everyone always wants to know what the “next big thing” is going to be.
I learned this bit of conventional wisdom from Clinton Dreisbach way back in the day. His version is “The third session is always the best”. It informed my design thinking about Grey Ranks!
One of the groups I game with mostly does a mix of one shots and 2-3 session arcs. We mostly do this for scheduling reasons and to be able to try lots of games. I’ve preferred to schedule 3-6 session arcs for games I run. I used to play in campaigns without a planned end date, but at this point it’s hard to get people to commit to games that will run a long time (or indefinitely).