Our work computers were on Big Sur and just today there was a mandatory upgrade to Monterey 12.4. On my 2019 MacBook Pro the upgrade took well over an hour, including a long time for the 12 GB download (at a relatively slow speed due to the servers, our internet is much faster), with multiple reboots, black screens, and at one point sitting at "less than1 minute remaining" for at least 15 minutes, followed by "37 seconds remaining" for another 20 minutes. I decided to pay attention to other things because watching this was making me anxious. When I returned some time later, the update had finished. I have heard that for some this process takes less than 30 minutes, but others report 3 hours or more. I wold leave it for at least several hours before resorting to a force shut down and reboot.