I've had multiple campaigns go 2 years, and a few that pushed 3 years. (Not sure if we hit 3 or not, but it was close.)
My preference is that a "campaign" run about 18-24 months, but I consider a level range part of what separates a campaign. My current D&D campaign will wind up when the characters hit about level 10, after 2-3 years. We probably will use the same setting, and the same characters, but it will be a new campaign as far as we are concerned. Might be some expanded options on characters, tweaks to house rules, that sort of thing. I'm loathe to change the parameters of what's in or out mid-campaign unless there is a real problem to solve.