20 - 30 minutes
My daughter in law had two 3GS iPhones. One had a broken screen, the other a battery that would not hold a charge. It seemed to me that the easiest fix would be to put a new battery in the phone with the phone with the intact screen.
Success! As usual, your instructions are impeccable. I am, however, amazed that I kept track of all 10 of the watch sized screws.
1. The suction cup method of opening an iPhone is almost too effective. The glass pulls off so easily that you need to take care not to break the three still connected ribbon cables.
2. Reassembly is easier if you find a way to immobilize the rear case. Gaffer tape works. A small bench vise would probably work better.
3. There are two particularly difficult steps in reassembly: the screw and clip that hold down the camera (Step 12) and reconnecting ribbon cable 3 (Step 6). Scotch tape is helpful with both. In step 12, align the camera clip on the screw hole and hold it in place with a bit of tape; then insert the screw. In step 6, hold ribbon cables 1 and 2 out of the way with tape as you fish cable 3 into place. Immobilizing the top case (perhaps by an assistant) would also be helpful here