20 - 30 minutes
To be honest, there wasn't any problem regarding my iPhone 3GS, it was working well. However, the power button seemed a bit "tired", and my battery didn't last as long as before.
Overall, it went very well. I had trouble opening up the phone, and I had to remove the screen protector I had placed, and then use the Section Cup to open up the phone. I also had trouble with the "Do not remove" adhesive, it tore apart into small pieces. Everything else regarding disassembly and re-assembly went well. However, I am disappointed about the replacement power button. It simply don't work. It didn't clicked when I pushed it. I kept the old one, and when comparing the two, it seems that the one IFixit sell is some cheap chinese copy. The Apple one has totally flat back plastic which allows to press the electronic part as it should, when the other one has a back plastic filled with holes and industrial marks. On a good note, the battery is as good as the original one.
The screws looks like they are all the same (except the two from the case). I recommend writing on a paper "Yellow, Orange and Red", and put the screws on it according to the repair guide. It really helps when re-assembling.
iPhone 3G and 3GS Power Button
iPhone 3GS Replacement Battery