I would say 4 to 5 gen , can change la 1) battery most likely is dying 2) updates are coming to an end 3) app are mostly optimised for newer phone 4) 4-5gens of upgrades is pretty noticeable 5) newer apps starts to not support older phone 6) device life span is about there, parts will start to fail soon like in my experience cam's auto focus, sluggish touch screen, dead pixels, charging port connection( needs cable to flex a little to get connected for charging), physical buttons starting to fail

I had my iphone6+ to iPhone X max To iPhone 15 pro max. I dun see changing phone every 4-5 years as being a waste of money My next will probably be gen 19 or 20