I can't remember the details off the top of my head anymore but I'd say 2-3 years twice through my 20's. Oh and why - the first time, I just wasn't interested in being with someone that wasn't interesting enough to change my life for.
Your 20's are the best time to be single and the best time to figure out just who the hell you are so if you constant attached to the hip (or married with children) with someone there is not possible way for you to reflect on you.