I don't know what Apple's iOS feature roadmap is, but I'd say there isn't really a rule of thumb you can apply.
My iPhone 6 struggled mightily with iOS 11 (even with an upgraded battery). It is the only time I have regretted an upgrade (since iOS 3) across 3 devices.
On the other hand, Apple has steadily expanded the number of years they provide updates for their hardware, up to about 5 years now.
My iPhone 6 struggled mightily with iOS 11 (even with an upgraded battery). It is the only time I have regretted an upgrade (since iOS 3) across 3 devices.
On the other hand, Apple has steadily expanded the number of years they provide updates for their hardware, up to about 5 years now.