Even when Apple was using Intel chips, they were updating at least once per year, sometimes twice per year. It was only when Intel crap the bed on 14nm that threw Apple's updates out of balance.I always assumed that we would be seeing a 2-year upgrade cycle for the M-chip because Apple doesn't sell enough Macs to make an annual refresh worth the R&D (unlike their iPhones). There's also the question of whether we will see enough of an improvement year on year for it to even make sense.
They don't sell their chips to anyone else either, and people are holding on to their devices longer also. We see it in the iPad lineup, where the Ax series chips typically get a 1.5 year refresh cycle.
Moving forward, I won't be surprised to see their processors get refreshed every 3 years. As long as Apple continues to lead in terms of power efficiency, I don't think they are too worried about intel / AMD chips being slightly faster in the high end, especially when the only way they achieve this is by consuming massive amounts of power, which means that they will never work in the form factors that Apple traditionally favours and currently holds sway in.
2 years is too long. 3 years is inconceivable. Even AMD's Zen gets updated once every 18-24 months.
Remember that it's the iPhone chip that makes it "affordable" to produce the M series. It's just a scaled up version of the A series.