That Mac and OS are recent enough that you shouldn't be having much trouble.
The time remaining estimates can be very misleading and often can change dramatically within a short span. It's my experience that internet connection speed greatly affects the estimate. I would allow the process to continue for a bit, but if it drags on any more than about an hour, 90 minutes tops, consider forcibly quitting the process by holding the power button until the computer shuts down.
I hope you have a Time Machine or other backup of your user files and data, otherwise this may turn into a learning experience regarding the pitfalls of not backing up.
Did you start this process by booting the Mac in Recovery mode using Command-R at startup?
Use macOS Recovery on an Intel-based Mac - Apple Support
How to reinstall macOS - Apple Support