20 - 30 minutes
The volume button had fallen off and the power button was non-functional
As this was my first iPhone (3GS) repair, the process took a long time (approx. 3 hrs?). Battery replacement on 2 others took 45 mins, then 20 mins.
iFixit guides were good but they could be great:
Step 4 could suggest running the spudger UNDER the black ribbon labeled 1. This make is a lot easier.
Step 6 was the most difficult to understand since the picture doesn't show enough detail and the tab that lifts up is so tiny. I could not get the tab to lift up. I ended breaking a small piece of the plastic past the tab. Luckily it had no apparent effect. I would suggest a simple drawn diagram showing the location and position of the tab before and after it is lifted.
Also, one washer came off of one screw, but I had to idea which one.
iPhone 3G and 3GS Volume Button
iPhone 3G and 3GS Power Button
iPhone 3GS Replacement Battery