how far into the future will iphone 16 and 15 be eligible for updates

Apple doesn’t have a guarantee on how long a particular model will continue to receive updates/upgrades. But a look at the past will generally give us a clue to future performance. For example, an iPhone Xr I have received the most recent upgrade, iOS 18. The Xr was released in October 2018. That was a full 6 years ago.

However, 6 years is a very long time for us dinosaurs. The Xr is limited in regard to Apple Intellegence and other new features. I know, dinosaurs aren’t interested in AI, but i felt it necessary to mentioned that some features may not be supported in 6 years.

Personally, I think you’ve hit the sweet spot. I suspect 5 years is a pretty safe bet, 6 is a 50/50 chance and 7 is a long shot, but could happen.

5 years is a conservative expectation based on past history.

Nobody would know of Apple's future plans. iOS 18 supports phone models. as early as 2018.

https://iosref.com/ios

How long is a long time? Three years, 5 years, 10 years or how long exactly?

You’re welcome and Happy Holidays!