I just realised my current phone is 21 months old. When I bought it the carrier very kindly allowed me to take 24 months interest free credit to pay for it, so I figured I'd use their capital instead of mine. They add 1/24 of the cost of the phone to my bill every month for two years, then that part of the bill goes away. I have three payments left.
The previous phone was the only one I replaced when it was still working perfectly, because I wanted to move to a new carrier and they didn't support my old phone. I think it was four phones ago I had one I grew to hate. One day I literally threw it against the wall in frustration with it and was quite disappointed when I picked it up and found it still worked. Although I said earlier I tend to replace phones when they stop working, if I get annoyed enough with a phone to do something like that with it the chances are it's getting replaced before too long.