2 - 5 minutes
I broke the rear panel of my iPhone 4 early this year. At first, I replaced it with a low cost aluminium panel purchased on Amazon. The result was aesthetically decent but the camera became unusable. All pictures were slightly blurred and presented vignetting. That's when I decided to buy the high quality transparent rear panel...
The repair was absolutely flawless...
Don't go for cheap replacements... ;-)