For me....
My longest : 9 years (+2 of dating/engagement) to Mr. AChiOAlumna...like someone else on this board said previously...how he puts up with me sometimes is a wonder....but he's wonderful, so we'll keep him around!!!
I also had a 4-1/2 year on again/off again relationship that lasted from my sophomore year of college until after graduation...
I had a 9 month relationship in my senior year of high school...
In between, I had a few 1-3 month relationships and a lot of dates...