Shackled City took us about three years. Same for Age of Worms. Our SWSE campaign died early, after maybe three months.
3.5 Night Below campaign went over three years, but I'm not sure how long exactly.
My M&M campaign is struggling due to non-overlapping cancellations, but I think it will stabilize and run several years. My 3.5/Pathfinder Beta campaign lasted about 4.5 years.
My longest running campaign was a Shadowrun campaign back in the early 90s that went about five years before ending in a TPK in "Universal Brotherhood."