Depends on the finance and country you live in. For where I live, we can do monthly contract and upgrade plan which lets you upgrade to new iPhone with a small monthly fee.
But looking for line up, I would say 3-5 years are good. iPhone X to iPhone 11 Pro Max is first gap. iPhone 12 Pro Max to iPhone 15 Pro Max would be another gap. I would say 15 pro max is the best upgrade to make, comparing to other models. 14 pro max was very heavy and not much change. Same goes for 12,13 pro max
We wouldn’t have a notch and something better like smaller or even invisible UI to access and control. Also, we would have no bezels very early like say iPhone 6 or 7 with how he progress.
But there are different what ifs, let’s say he was here today, Samsung could be taking over the mobile instead or Sony might still be dominating the market. I guess you never know.