2 - 5 minutes
I dropped my iphone on a patch of carpet, and the back glass decided to break. I went a year without bothering to fix it.
Great. Took 5 minutes, literally.
Don't eat fast food before changing the glass, or you will go mad looking at your own fingerprints, especially on the inside of the new glass.
Phillips #000 Screwdriver
P2 Pentalobe Screwdriver iPhone
iPhone 4 Revelation Kit (GSM/AT&T)