Well, technically speaking, the Traveller campaign the EOW group has been doing has been going on for years, like maybe seven.
On the other hand, they only meet four times a year.
And I only make one of those sessions, at the annual EOW mini-convention. My PC is a crew member who works in the loading bay of the main commercial spaceship the group owns, so that's not really a big deal when I'm gone.
So, on the side of "this counts and should be my answer" the EOW Traveller campaign is one that has seen the group go from being the crew of a plucky little Free Trader called unimaginatively Beowulf to being about the Beowulf Corporation, which commands a small fleet of ships (mostly acquired through salvaging disabled pirate vessels). The campaign has a rotating GM roster, and I heard that the last session's GM sort of hit the "reset button" and launched the core crew into another universe where they lost all their resources, et al and are basically back to square one. We'll see how long that lasts.
In terms of actual playing sessions? I don't know. I'm terrible about running campaigns long term, as Adam and I have kicked around here before. Maybe the D&D campaign I did four or five years ago that went from 1st to about 15th level. I just did a quick check of the "Friday Night Recap" posts for this 5E game I'm doing at we around 19 or 20 sessions on this one (and at 6th level) which is pretty damn good for me, given my track record. (This would be, he says in a joking way, about sixty hours of game time, or only seven sessions in Adam's universe, which isn't much at all.)
I could go on at length about how really difficult it is for me to run a campaign over a long period of time. Maybe another post.