View amount of time left when uploading files to iCloud
The way to check all percentages is:
brctl monitor com.apple.CloudDocs | grep %

Possibly some other commands will help as well if the above has too much detail. Open terminal.app and issue the command:
brctl log -w

That will dump all sorts of iCloud sync data and debugging status - most of which won't be relevant, but you can watch it over time to see if any of the internal information allows you to calculate things or just know that something is happening or has had an error.
brctl status | grep foreground

The above command will show active containers and you might be able to monitor the specific container that has activity in a second window:
brctl monitor com.apple.CloudDocs | grep filename

The monitor command could slow things down as it dumps a lot of status to my screen as I have a lot of files in iCloud currently - but it does show periodic % estimates for the specific files being uploaded. Change filename to match a file or folder you want to check on.
All of the commands that run indefinitely above can be quit with Control + C