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

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

https://iosref.com/ios

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.

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

5 years is a conservative expectation based on past history.

You’re welcome and Happy Holidays!