I think the best way to go about this is to look at exactly what they’re doing right now.
Both iOS 16 and Ventura require a Mac or iOS device released on or after June 2017.
If Apple continues this pattern, then we should see next years update support 2018 and later, the year after support 2019 and later, and the year after that support 2020 and later.
That would put the first all Apple silicon operating system in around 2025/26, depending on their cut off priorities.
Also, there’s a major possibility they don’t stick to the schedule, there have been several versions of iOS and macOS in the past that have retained the exact same support list from their previous version.
Mountain lion, Mavericks, Yosemite, and El Capitan were four versions of macOS in a row that didn’t drop a single device.
Same with iOS 13, 14 and 15.
Safest bet is to use what you have now for as long as you can, and by the time they do start dropping Intel support, you should find apple silicon computers for more affordable prices.