2 - 5 minutes
I dropped my iPhone and the top left corner completely shattered. I didn't want to pay $50 for a new black rear panel so I chose the transparent rear panel.
It was a very easy repair. It took about 3 minutes. It looks amazing on my iPhone!
I would recommend that you spend a few extra dollars to buy the regular iFixit pentalobe screwdriver rather than the one in the iPhone 4/4S Liberation kit because it will fit better if you have pentalobe screws on your iPhone.