Probably the longest single campaign I initially ran started in 1E, and was converted over to 2E. The primary campaign used A1-4 Slavelord series, I6/I10 Ravenloft/Griffon hill (simultaneously), and a heavily modified/homebrewed G1-3 Giants and D1-D3 series, culminating in Q1. The campaign lasted for over 3 years and had a few additional adventures after the Q1 wrap-up. The characters were around 14th level, and each of the PCs were coming into their destiny (An elvin king, a paladin Baron, a samurai Daimyo, a monk Master, a clerical Pope, a ranger Lord, a wizard Archmage and a halfing Syndicate boss). The players were ready to retire their characters and I was burned out.
Years later, we did try other adventures - and finished one, but that group was pretty much done. A couple of years ago, we started up another game, a mix of the original players and their actual children and we played a short campaign starring the character’s offspring.
That’s only one campaign of several I’ve run or played in over the years. I’ve been in short and long campaigns, but we have always tended to wrap the game up no later than somewhere in the teens, and I have not been in a campaign on either side of the screen that has made it past the levels of the first campaign I indicate, and I’m fine with that.
And just to note, the campaign I’m running right now (Saltmarsh) has ties back to a 3E campaign I ran in which the latter characters were pirates that ended shortly after the group completed their primary campaign-long objecting of recovering Bloody Jack Dascombe’s cursed treasure (and that campaign ended around 12th-13th level). So, while I don’t continue a single campaign forever and ever, I often tie in events, characters and places from prior campaigns for the players to interact with (“Hey, I remember those guys from the last campaign!”)