The sweet spot would be putting a new battery into a 3 year old phone to get another 3 years out of it.
So 7 years of updates would be fine, maybe with one battery swap in the middle, as long as Google starts paying a bit of attention to their phones' quality assurance and control.
I'd personally like to see swappable storage and mandatory external SD card support.
We could probably get to 15 years with current phone tech, unless they invent something really revolutionary that makes everyone want to upgrade.