My previous Macbook was a Pro model from 2015, I waited 6 years to finally upgrade to an M1 Air because of the awful touchbar models they had in between (though I'm still using the 2015 Pro for personal stuff, in fact right now. It's upgraded to the latest macOS using OpenCore and it still runs great). But I would say upgrade every 3-5 years depending on heavy a professional user you are.