If you want the crystal ball version:
The 11 Pro Max was issued originally with iOS 13. In the general run of system release times and model support over recent history, an iPhone has typically supported 4-6 major system versions. So let's say 5. That would mean 13 + 5 = iOS 18.
iOS 14 came out last September. Apple has been doing annual iOS releases recently (that's not to say they couldn't change things, and they have in the past). That would give the 11 Pro Max about another 4-5 years or so of iOS updates. This would be in line with Apple's hardware support statement of 5 years:
Obtaining service for your Apple product after an expired warranty [was: Vintage and obsolete products] https://support.apple.com/HT201624
As I said, from my perspective even being behind an iOS release or two probably won't see essential deficiencies unless you absolutely must be up with all the latest stuff. App developers often support the last 2-3 iOS versions and I am still seeing updates to most of my iOS 12 apps in the iOS 14 era.
I would say that if it does last 5+ 2 iOS generations you'll be lucky. We have found that after about 5 years or so there's some hardware failure that retires our iPhones.