My campaigns vary, but they typically fall in the 2-3 years though more closer to 3 years. I would like them to be more on the 2 years kind of side, but I find that my style, XP award management, and the games I run, in order to players from 1 - 20+ levels, it's at the 3 year mark for me.
My current campaign has already ran for over two years now and I think it's got about six months left before I wrap it up. This time I expect it to go to about 15th or 16th level.
My next campaign will be a short one, only 5th - 10th level which will be a Sandbox Reverse Dungeon Evil campaign (my players threw out an odd idea at me for the next one). However, I think that will last a year.