My current campaign extends back to the mid-90s, albeit with many different groups of pcs, and ties to my old Oerth campaign (in which Tharizdun ate the multiverse) via time travel related shenanigans. That game started in, hmm, about 1981.
Typically, I run a given group until we're done with them- either via TPK or some kind of natural conclusion. 4e is the only version that actually had a built-in end point for me; various 1e pcs went up to levels in the 20s (and in one case to somewhere around level 43), 2e pcs reached into the high teens or maybe low 20s, 3e pcs came into the high 30s, and my 4e game wrapped up around 27th to 28th level.
Of course, not every group of pcs goes all that high, and often I'll run multiple parties for the same players concurrently, and we'll periodically switch back and forth. Sometimes they'll end up mashed together; at one point, after running two-sorta-three different groups of pcs for a while, I had each player choose one for the next phase of the game.
But yeah, I definitely consider it all one campaign. It's all the same setting (well, plus the Oerth stuff, but it's all tied together on a very deep level that pcs have influenced and interacted with in major ways), there are certain broad themes that have run through it, and the pcs have made lasting impacts on the world, both good and ill.