FYI Steve jobs announced the transition to Intel in June 2005, said it would take about two years (sound familiar?) but the transition was actually completed 14 months later. Snow Leopard was the last macOS version to support PowerPC, and perhaps because of that it has substantial longevity and was supported/updated 2009-2011.
(Conceivably Apple could do something similar if a geographic area like the EU has a mandate on OS support to match their hardware support requirement.)
So the lifetime could conceivably be more than 3 years, but practically - with the introduction of USB4/Thunderbolt4 likely only on ARM machines, plus the addition of AI cores (as in A-series chips used in iOS), plus the promise of dramatic speed increases unlike what we’ve seen the last several years with Intel - people, especially MacPowerUsers™, won’t want to hold onto new hardware for longer (or even that long).
I’m currently a bit at a personal impasse as I learn more about the potential power of the new ARM hardware. I’ve long been planning to get a new 27" iMac this fall to replace my 2017 Retina iMac, but if the power of an ARM/iMac is is the ballpark of Intel, then given the promised (though not fully explained) ability to run iOS apps on Big Sur/ARM, I might well consider going that route earlier than planned. (The main complication for me is that the rumor is that we’ll see a 24" ARM iMac only in 2020, and I really prefer the larger Retina screen.)