Let me start by saying that I bought my iPhone 3GS in October of last year and have no plans to upgrade to the iPhone 4. Luckily, while the new iPhone--antenna issues aside--has certainly been enhanced, the differences between the new model and my "old" 3GS aren't huge, especially now that I've installed iOS4. However, even if there was a big difference, I knew going in that I would largely be ignoring whatever next-generation model Apple put out because I'm not a serial upgrader.
But recently I've been thinking about how long a phone (and not just an iPhone) should last. The common consensus I've heard from cell phone manufacturers is about two years, even if some carriers allow you to upgrade every 12 months (at a subsidized price), depending on your plan. With my old phone, the Sprint Mogul, which was made by HTC, I actually logged more than two years, and fellow editor John Falcone has remarkably stuck with his Mogul for over two and a half years as he awaits the arrival of the perfect phone.
I know plenty of people who've held on to the original iPhone and its 2G service, but most of those people (the ones I know anyway) aren't heavy data users. I would have moved on if I owned the first iPhone (I didn't buy it), but I'm thinking the 3GS could do me just fine for well over two years, though I'd probably have to replace the battery, which isn't that difficult. (See Rick Broida's article on replacing the iPhone 3G's battery).
Phones like Sprint's Android-powered Evo, which truly uses a 4G network, should also be able to put in a solid two to three years of service before seeming too ancient. And if all you care about is making voice calls, your phone can theoretically last for years. (In fact, I know one guy who five years ago received a flip phone in a goodie bag for an event. It came with free service that the carrier has forgotten to cut off, so he's never upgraded the phone.)
Of course, your phone can meet an untimely end in a number of ways. Death by dropping, drowning, abandonment, or plain old thievery seem to the most prevalent forms of demise. But those who take extra care with their phones, protecting them with a case and making sure they don't fall out of pocket, can help them stay in pristine condition and lead longer, healthier lives (I don't know about happy)--and have a higher resale value should you decide to sell on eBay.
How 'bout you? How long do you hang on to your cell phones?