20 - 30 Minuten
The battery in my iPhone 3GS was going bad. It would turn off with out warning around 50%.
It took longer then I thought it would. The small "00" size screw driver that came with the Battery kit wouldn't work on 8 of the 10 screws that I had to take out. I had to find another small Phillips that would work. I think it was the point of the driver that was too long/deep and it wasn't getting a good enough bite to break the screws lose. The written instruction were okay but the video was pointless since it was just promo for selling the product and didn't help with the real tear down.
I think more info on putting the phone back togather would be good since this for me was the more difficult part i.e. getting the logic board to line up and seat correctly. It took about four try's and then I wasnt sure it was going to work.