How many years the iPhone Xs max last

Only Apple can say for sure, to find out call the 1-800-MY-APPLE to speak with tech support, or use the Support App from Apple to chat.

They will know for sure on the latest iOS update available for your device.

Here’s my swag; scientific wild *** guess

So if I understand the question, that how long with phone last after the last update and you've been able to load 16.1.1 to your XS today?

if so, my guess on how long the iPhone hardware will continue to run would be;

Providing you don’t cause any hardware damage by dropping, cracking screen, etc…

Four or Five years, and possibly longer if you take care of its battery. I still have one of those, it still works.