It depends, but it is usually pretty fast.
You can check it on GOS release page when the build was available for specific phone:
For instance release 2022101600 was already for Pixel 7 and Pixel 7 pro.
That release number means the date: 2022 10 16
That phone was released couple of days before:
So the build for the new phone was available within a couple of days.
This is not guaranted, but it seems that GOS team does their best to support the Pixel phones as soon as it is possible.