A while ago I asked you what was first smartphone you purchased, and now I want to branch out along the same line and ask how often do you buy a new phone?
I've developed a pattern of purchasing a new phone every 1.5-2 years. I don't upgrade every year as the yearly improvements from one generation to the next are never worth it. Instead I tend to skip one generation and buy a phone from the next generation, so my phones are always two generations apart. When I buy a new phone it's never out of necessity, but desire. My phones are always working fine when I upgrade, so I either sell it off to recuperate some of the cost of my new phone, or I hand it down to my siblings.
How long do you keep a phone before buying a new one? Is it normally out of necessity or desire? Do you upgrade via your carrier? What do you normally do with your old phones?