I'd say it depends on how often the technology goes out of date.

First, whatever-it-was to edge, then edge to 3g, and now that I have an iPhone 3GS, I'll probably retire it either when the toddler finally breaks it or apple decides to stop supporting it with OS updates. At this point my phone cycling schedule would probably be the same as Apple's since I like the iPhone interface and thus have no intention at this time of switching over to a droid or blackberry or whathaveyou.