Well, the longest I've ever had a phone was the iPhone 3G, had that the full 2 years until the iPhone 4 came out.
As for shortest, not sure if this counts, but I got the Samsung Galaxy Mega last year and had it for 4 days before returning it, it was just too big. Other than that one, or other phones I returned during the return window, it would be the Moto X. Now don't get me wrong, the Moto X was an awesome phone and I do miss it somewhat, but I was offered a Nexus 5 32gb for it and I couldn't pass it up. Had the X for about a month.