Day 25: Longest running campaign/gaming group you've been in.
I ran a campaign in my own setting for 11 years. It chronicled the attempts of the players to stop a servant of Orcus from taking over the world with his undead hordes. The campaign started with 1E AD&D and moved to 2E when that came out. The majority of the players were there from the beginning to the end. The characters actually managed to defeat the bad guy after 10 years of play time, with the last year seeing the heroes play more one-shot adventures within the world setting. However, without the drive of the singular plot, interest waned...or we were just tired of it.
I did use some of the elements of the setting a few years later when we began to run some LARP (live action role-play) events. These events saw the main villain's spirit being reborn on another world (the LARP setting) and attempting to take over that world as well with undead armies. One of the original players even created one of his LARP characters as the son of one of the original characters from the D&D campaign.
I tend to prefer running longer games rather than one-shots. I prefer the more epic feel in such campaigns where you can see the characters and setting grow and evolve. However, I don't think I'll run an 11-year campaign again.