Nobody knows for sure, but here's something for an educated guess:
- Previous transition took from 10.4 to 10.6 to drop PPC
- Apple generally speaking supports old hardware for three years
So if we have luck, Big Sur and Monterey are the macOS that have Rosetta, and The Next Place introduced next year may drop Intel support altogether.
... and given it took 3-4 years to get the Mac Pro right last time, they may want to wait at least until macOS is Apple Silicon only to release "ARM Pro" workstation.
Just guessing though.