I have enough anecdotal evidence of people still using old iPhones all the way back to the terrible 3G :)
What we do know is that about 80% of iOS users are on iOS 8 (=iPhone 4S or newer), as per Apple's statistics or this website:
https://mixpanel.com/trends/#report/ios_8/
And models older than the iPhone 4 seem to very rare:
https://mixpanel.com/trends/#report/iphone_models
So I guess most people update their phones at least once every four years.