I have run into similar situations with previous OS installs on other Macs. And after several hour with the installer saying ___ minutes remaining I finally gave up and held down the power button till the system shut off.
Upon powering up it finished the install process and went through the new OS post-install screens that ask to verify settings, iCloud, etc. And left me at the login screen, which worked fine.
HOWEVER I hope you have a backup in case my use-case scenario was atypical. So proceed at your own risk. But honestly you don't have much choice. The system is likely hung on something that could never time-out. So you really don't have another choice, you'll have to power cycle the Mac and hope for the best.