Apple announced the new iPhone SE (2nd generation) which is bigger, about the size of an iPhone 8, with a 4.7" screen. A13 Bionic chip, same as in iPhone 11, though how many cores or speed is not noted. One 12MP camera back, 7MP camera front, Touch ID, up to 256 GB ($550).

I bought my iPhone SE June 2017 but it was completely replaced October 2019. So I think it should last me two years before the battery starts degrading. If I can keep it for another two years (unlikely) then it'd be nice to wait until mid 2023 for a new iPhone.

Hmm, I bought my current iPad September 2017 so it'd be bad to replace it in the same year as my iPhone. Let me think...

iPhone SE - bought Jun 2017
iPad Pro - bought Sep 2017
MacBook Pro - bought May 2015
Mac mini - bought Feb 2018

Replacement Schedule

MacBook Pro - mid 2021
iPad Pro - mid 2022
iPhone SE - mid 2023
Mac mini - mid 2024

So 6 years to replace except iPad Pro which will be in 5.