For iPad, Watch and Macs I tend to hold onto them until after the final Security Update is released. Where in I buy the the next future model that directly replaces it.
Example: 2012 iMac 27" 22nm to 2023 iMac 27" 5nm. Final Security Update was 5 months ago and I hope a iMac 27" replacement will be out by WWDC 2023 in June. I also expect power consumption to drop from >200W to <100W when I replace this 10yo iMac.
Reason being my use case has not changed since 2015. If you have not changed its use then why follow industry studies that say replacement is expected in 4-5-6 years?
With the iPhone I stick to every 2 years for the "S" model. I just want a better camera, fresh new battery, better performance per watt and hardware that has not been dropped or jostled as I dont want to be holding a disabled phone in a possibly high stress environment.
Not to mention it's a business expense.