No one knows. Apple have said they will support Intel Macs for "years to come" but they haven't yet said exactly how many years that will be. And that's on purpose, i think that's a detail they may not have decided on and they will save that for later. I don't think they'll drop support super quickly. I don't think they're interested in leaving people hanging abruptly for this. They have the money and resources to support the last Intel Macs with software updates for the usual 7 years if they want to. My Late 2013 MBP will be getting Big Sur as it's last OS update. If they stick to that. A 2020 Intel MBP will get it's last OS update in 2027 with 2028 being the first time it does not get the new OS. I personally think they will try to stick to that as long as they can, but if ARM Macs enable features that can't work on aging Intel machines, they will drop support sooner if it makes the most sense. I would say maybe as early as 4 years, that could potentially happen. So i'm thinking 4-7 years for OS support.