Longest for me is my '05 Escape, which I bought used. I can't remember the exact date I bought it, but it's been about 12 years, I think.
Shortest is technically my current Mustang because, while I still own it and don't plan on selling it, I've only had it for a few months. Not sure that counts, though. For shortest total ownership, I'm not sure, I've owned a number of cars and usually keep them for a while. It was probably my '78 Ford Fairmont, which I kind of inherited from my girlfriend. We moved to California together in it, and then added a Fiero, and when we broke up she kept the Fiero and its payments, and I kept the Fairmont, and I think I had it for about four years. It was a rust bucket, and started having all kinds of problems, and I replaced it with a very sporty (
) Dodge Daytona Turbo.