Hiya!
Well, as an old curmudgeonly grognard here, I can honestly say that the concept of "when does a campaign end?" seems....odd. Probably because it's one of the things...top 5, maybe even top 3...that made me fall in love with RPG's back when I was a wee lad of 10 years old in 1980; "The game
never ends..."
I mean, I have a character. Denakhan. He was 'born' in 1981, shortly after I started getting my footing in B/X D&D. I played/DM (we were kids and kinda took turns DM'ing for the first year...then I kinda just 'took the wheel' as DM)...anyway...6 years of constant 'play' later, moving into 1e AD&D, Denakhan is a 20th level Magic-User with a "God-Curse on him to not remember/use any of his Fighter stuff [F/MU]. About 5 or 6 years later, I made new friends and all that. Picked up Denakhan right where I left him (although only as a Player now). A decade or so after that, did a 'three-off' with him (same DM, couple different players)...picked up right where he left off). Right now, I guarantee I could go to my best bud's house and say "Hey, DM me for a few hours. Just wanna roll some dice and whatnot playing Denakhan", and my buddy would pull out the screen, and start with "Last time we played...I think you and Pic just exited the Bath House Dungeon in Birarwood, right? Oh, wait...no. It was a couple days later and that Brown Dragon with the disease-puke-breath showed up and you guys chased it off. Ok. Lets start...". Last time I played Denakhan? About 1997 or so I think?
So...it's the "same campaign", with the same characters, and the same timeline. It never "ends". We've made other PC's and played in that world and timeline. Different stories and whatnot...sometimes hearing of the exploits of our other PC's. It's what makes a "campaign" an actual "Campaign"....and not just s group of adventures that don't matter.
I guess the answer to "How long do you like your Campaigns to last?" would be: Forever.
HOWEVER...if we are just defining a "campaign" as nothing more than a series of adventures with the same 'story/PCs', as a self-contained "one-off campaign"...so a beginning, middle and definitive end... I'd say....two years. That's about right I think. Should be enough time to get a 1e PC from level 1 to about level 10 to 12.
^_^
Paul L. Ming