When I can't play a new game I want to play, I'll upgrade. This varies. My last computer i7 920 with a GTX 470 lasted me for a long time -- around 9 years. I have a Ryzen 2700x with a 3060TI that I built in 2018 and added the newer GPU in 2021. I'll probably upgrade next year so around 7-8 years. Before that I had a Pentium 4, Pentium 2, Pentium 1 so those are roughly 4 years between but progress was more impactful back then.
Averaging things out -- I'd say 6-7 years between major builds.