2 - 5 Minuten
So I was fishing my naked iPhone out of my pocket, and I lost hold of the phone. It went flying about 10 feet in front of me to the asphalt. The back glass was shattered but the phone still functioned.
I simply removed the 2 double-zero Phillips to the right and left of the USB connector, slid off the broken back glass and slid on the new back glass, and reattached the screws. It took about 1 minute total.
Replacing the back glass of the iPhone 4 _does not_ require the 5-point pentalobe screwdriver.