Time Machine drive encryption going for 22 hours. Is it stalled?

22 hours is very long indeed. I have encrypted a 1TB drive in less time before.

The encryption is handled by a "live background process". "This process continues seamlessly across reboots. The logical volume remains usable at all times."

^{from man page > diskutil > encryptVolume}

Maybe this daemon hangs or simply halted because the drive is not mounted (which is fine). So you should just re-mount it again.

Alternatively, I would try I restart the daemon by simply restarting the computer. Once the Time Machine backup is mounted again, the daemon will continue encrypting the drive.

Checking the encryption progress using diskutil

You can check the status of the encryption by using the command:

diskutil cs list

In the nested tree you will see a Logical Volume Family which is currently being encrypted. If you drive is being encrypted, you should see the entry Conversion Status: Converting .

Encryption Status:           Unlocked
    Encryption Type:         AES-XTS
    Conversion Status:       Converting      //what does it say here?
    Conversion Direction:    forward
    Has Encrypted Extents:   Yes
    Fully Secure:            No
    Passphrase Required:     Yes