Probably the longest campaign I ever played lasted just over a year-and-a-half. It was a D&D 3.X (we transitioned in the middle of the campaign between 3rd edition and 3.5) taking place in the World’s Largest Dungeon. When you play a published adventure for such a long time, you become acutely aware of editing mistakes and inconsistencies in the text. You also find very quickly where the limitations in the source material lie. For example, WLD was written in the early days of 3rd edition. By the time we started, several “splat-books” had been released, greatly expanding the options available to characters and my policy for this campaign of “anything goes” (that’s the last time I ever did that!) resulted in characters which were way above the power level the designers expected. As a result, many encounters, particularly in the early parts of the dungeon, were way too easy for the PCs. Some of my players at the time were hardcore min-maxing power gamers, as well. Still, we stuck with it and by the end of the campaign, several players had develop some very bad habits, like kill first-never talk. It took YEARS to break some of them of that.
I haven’t been able to stay interested in a campaign that long since. I think it broke me.