In the two campaigns I've actually played in, only one of them actually had an ending; the other just sort of faded away.
In the games I've GMed, it's a worse percentage; of the seven or eight games that lasted long enough for me to count them as campaigns, only two ever made it to a conclusion; all of them fizzled out from lack of player availability or GM interest; the two that succeeded were especially fun, though, and one was a 2-month, weekly Star Wars game, and the other was a 4-month bi-weekly fantasy game.
Generally, I can't keep a hold of players long enough to run a truly successful game; some have gone off to college, a couple have gotten full-time jobs, and so forth.