It should probably only take 10 to 15 minutes at the most, but there is a problem with the factory reset feature. I've had two 920s that got stuck on the spinning gears when I tried it. The first time I was within the return window and exchanged the phone; the second time I had to flash the phone myself to get it working again.
If your phone gets stuck during the reset you can either contact Nokia Care and send it in or look for the stickied thread at the top of this forum that has instructions on how to un-brick a 920.