2 - 5 minutes
This was the second time I have had to replace the back panel of my iPhone 4 GSM. The first time I dropped my phone while crossing a downtown street. The second, I was wrestling around with my dog and rolled on top of it.
The transparent panel seemed to be of less quality than the first one. There was no lens cover, and it wasn't properly glued in one of the corners. Following the video was better than the written instructions. The written instructions didn't include details like switching the vibrate mode before proceeding.
I didn't know they came out with a mirrored rear panel... It looks cool.