Why my macbook is taking 9 hours to update from big sur 11.5.2 to Monterey 12.1?

The install does NOT require an internet connection.

Do you have enough free disk space? You need a minimum of 26GB.

Uninstall any antivirus and "cleaning" apps using the apps' instructions. since they sometime cause updating problems.

Try booting to Safe Mode. That boot up takes minutes and clears caches and other things that may resolve your problem.

How to use safe mode on your Mac - Apple Support

Once in Safe Mode delete the existing installer and redownload the installer and try installing.

I'd guess very slow internet connection, web congestion or just one of those things. Sometimes processes on all our Macs will be unusually slow for no apparent reason. Often stopping the process and restating it is enough to make it work normally.

If that doesn't work, do a computer restart. If that doesn't help startup in safe mode and let it sit for 10 minutes and then do a normal restart.

Safe mode: https://support.apple.com/en-us/HT201262

The time it takes may depend if FileVault was being used while running an out dated version of Big Sur ( current Big Sur is now 11.6.2 ).

Had the drive not finished Decrypting before applying the Monterey upgrade - that could be a factor.