Usually around 2-3 years. That's usually around when my current phone stops receiving software updates and by then I can find a good deal on an outgoing model.
For instance I upgraded to a Pixel 7 Pro when the 8 came out. I sold my 6 Pro for a decent amount and my total cost of upgrading was something like 300 bucks which was worth it to me to not have to worry about it for another few years.
Thanks for the reply. It does seem to be a pretty incremental upgrade from what I've read. I've been really happy with my 6 Pro so I'll probably continue to run it for a while.