My experience for 10.8.1 and 10.9.1 were quite different.
For my 10.9.1 single machine installation, it had to remove 7 patches and took approximately 4 hours.
For my 10.8.1 single machine installation, it had to remove 15 patches.
It took 2.5 hours to uninstall "Portal for ArcGIS Security 2023 Update 1 Patch B" At that point, the machine rebooted itself. I restarted the process.
It then took an additional 2.5 hours to uninstall "Portal for ArcGIS Security 2023 Update 1 Patch A" and rebooted the server yet again. I restarted the process.
It took about 1.5 hours to uninstall "Portal for ArcGIS Security 2022 Update 2 Patch" and rebooted the server again. I restarted the process.
Finally, it took about 7 hours to uninstall the remaining patches (no further automatic reboots).
The overall time for 10.8.1 was 13.5 hours.
Then, the revised patches had to be installed. It took about 1.25 hours to install the patches on 10.9.1. It took 4.25 hours to install the patches on 10.8.1. No complaints from users on this Monday morning yet...
Hope this helps someone with understanding how long of a patching window to plan for.