I usually go every other year as I don't trade in my older device, but I did buy the 6 Plus and 6S Plus back to back. I skipped the 7 series and will probably buy the next model this fall. Some people upgrade every year. It just depends on what you can afford and what features you can or can't live without, as the latest phone will always gets a hardware/software feature that the previous models won't have.
What @JML5150 said. Depends entirely on your needs, wants, and pocketbook.
Before my current iPhone 6s Plus I had a Windows Phone for 3.5 years (Nokia Lumia 920) and would have kept it longer but Microsoft ceased support and it wouldn't officially upgrade to Windows 10 Mobile. I tend to keep my phones a long while. I'll probably keep my iPs+ till at least next year's (2018) iPhone offerings.