5 - 30 Minuten
The power/sleep button on top of my iPhone 4 had stopped working. It didn't click anymore, and only worked if you pushed down on it VERY hard.
The repair took some time--almost 3 hours, I think. You have to remove a lot of the insides of the phone to get to the cable. I got a little nervous looking at all the parts sitting on my desk, but they all went back in. Have patience.
I followed someone else's advice of putting the pieces (ESPECIALLY the screws) on a clean sheet of paper and labeling them. This was very helpful. If you wear reading glasses, I would recommend getting some kind of hands-free magnifier/light so you can get a good view of what you are working on. This was a very rewarding repair. The part was $20--a lot cheaper than replacing the phone.
Phillips #00 Screwdriver
Flathead 3/32" or 2.5 mm Screwdriver
iPhone 4 Replacement Battery
iPhone 4 (GSM/AT&T) Power and Sensor Cable