i kept my iPhone 3GS from 2009-2011 (2 years)
i kept my iPhone 4S from 2011-2014 (3 years - even installed a new battery myself).
i kept my iPhone 6 from 2014-2017 (3 years)
i had an iPhone 8 from late 2017-present, hated it, sold it and got an iPhone X.
I plan on keeping this iPhone X probably 3-4 years. It's the best iPhone ever made.
i kept my iPhone 4S from 2011-2014 (3 years - even installed a new battery myself).
i kept my iPhone 6 from 2014-2017 (3 years)
i had an iPhone 8 from late 2017-present, hated it, sold it and got an iPhone X.
I plan on keeping this iPhone X probably 3-4 years. It's the best iPhone ever made.