30 minutes - 1 hour
Two weeks prior to the IMac failing completely I noticed that the PC shut down and couldn't be turned on. I found some help on the internet that allowed me to reset the power supply. Pull the cord, wait 10 seconds, hold in the power button for 5 to 10 seconds, release, then plug in and power up normally.
The power supply failure occurred two weeks later so I ordered the power supply through ifixit and followed the procedures to replace it.
The ifixit steps were very clear. I used a bath tub mat with suction cups to get the outer glass off. The rest of the procedure went well. I did need an extra hand to hold the screen up while I disconnected the four connectors. The hardest part was making sure the glass was clean when I put it back together.
Be careful but don't be afraid of replacing the power supply. Next time, I would use individual torx screw drivers instead of the multiple tool torx set. The short multitool torx set is a bit awkward and I had to be careful that it didn't slip and hit the glass.