While this number varies greatly depending on how you handle a smartphone -- perhaps you're a construction worker whose phone will be put through the harshest climates or you have kids who may drop the phone ever so often -- the iPhone will typically run efficiently for three to four years. Then, you'll likely experience more frequent app crashes, lag, storage problems, and more. Those are common issues that any piece of technology will inhibit, not just the iPhone, after years of constant usage. If you'd like to prolong that time, I'd recommend visiting your local Apple store to have the phone's battery replaced every three to four years and also consider backing up files to an external hard drive for safekeeping. As a last resort, a factory reset, followed by data restoration, should give your phone a brand-new feel.