The longest-running campaign that I’ve played in is a Rolemaster campaign. Some friends of ours have been playing the same Rolemaster campaign for literally as long as Rolemaster has existed. Some of the same players have been involved for ±30 years now, and most of the current group has been around for 5-10 years. I’ve only joined in for a couple handfuls of sessions, spanning a few years, but this group has been going for roughly as long as I’ve been roleplaying, period.
Oh, and when you play Rolemaster that long, even with all the supplements and options (they’re still using the edition/books that were current when they started), you learn it really well. For example, the GM and many of the players have functionally memorized the endless spell lists, so what is horribly unwieldy to me is as easy for them as remembering that fireball is a 20’-radius blast. It was one of the easiest games I’ve ever learned, despite the detail and complexity, because everyone else at the table knows it so well that there was always someone available to explain something or answer a question about how the rules worked or what skill applied.