Yeah, maybe 2027. The M1 chip is essentially based on the A14 Bionic chip found in the iPhone 12 Series. So I'd imagine whenever Apple deems the 12 series obsolete might mean the M1 is on shaky ground. I don't see that for a while yet, seeing as the Xs just got the chop and the 11 series is the baseline for at least the next 1-2 years.Apple will drop OS support for M1 in 2027. 2020-2027 would be 7 years of support. There is no incentive for Apple to extend the life of those machines because Apple needs people to buy new Macs.
Apple has shareholders to satisfy, so it is a pipe dream to expect M1 to have a longer life just because Apple makes the hardware.
Apple could support the M1 chips for years, but just with an asterisk as it's not deemed powerful enough to run the very latest AI tech found in macOS in 2027/28, particularly devices with 8GB of RAM.
But it's just guesswork as no one knows.