I have the same issue when I did my factory reset, it seems to be a issue with some 920 where they get stuck. try the below to see if you can fix it but otherwsie it may need to get sent for repair.
Normally, it takes only 30 minutes at most to reset it. In the event that it would take longer, like what happened here, you can press and hold both on the volume down button and power key for about 10 seconds.
The phone should then reboot. If the phone would still be stuck to spinning gears, here is a hardware reset that you need to press. Input these keys on the following sequence, press them one key at a time:
Make sure that you try this while the phone is charged.