Most people upgrade their phones every two years half the voters in our poll do just that. It makes sense, most carriers lock you into a 2-year contract. And even if you buy a phone SIM-free, most phone makers are on a tick-tock upgrade cycle.
You know how it goes iPhone 6, iPhone 6s, iPhone 7... well, the iPhone 8 is 7s in all but name. Then theres the Galaxy S6 new design but with issues, which the S7 fixed. The Galaxy S8 brought a new design and in a couple of weeks, the S9 will refine it.
Then theres a nearly even split between people who change phones every year and those who do so every three years or more. That ties into the previous poll, which showed that most people expect 3 years of software updates.
If you change your phone two years after it launched, an extra year of software support will up its resale value. Alternatively, if you buy a phone late in its life cycle (when its cheap), then you know youll get two years of fresh software.