I think when that happens in a game it is never because it is too long, and always because it just stopped being fun enough. I think DAO could have avoided that by having more dragons and bosses nearer the end. My only IRL friend who played this game said he quit at the Dark Roads. He is so typical of the average player, I expect that happened to a loooot of people.DAO is really long with all the DLC.
Even without the DLC, I thought it was almost a little too long. I couldn't wait for it to be over towards the end.
EverQuest really proved to me just how much tedious grind people can put up with, almost infinite. But the game has to provide enough carrot and stick at regular points that are significant enough to coerce the player into actually wanting to push forward.