20 - 30 minutos
My 3GS was having battery troubles: it was wearing down much quicker than when I bought it and the phone would power off even though it supposedly had 20% battery remaining.
The repair went very well. It ended up taking about an hour, but I bet I could do it again in 20 minutes. The battery is now working like new.
Two areas where I had issues:
1) I wasn't clear on how to remove the connectors, and the photos weren't as descriptive as they could have been. I didn't realize that you really need to get under the ribbon cable and pry it up, and it pops off. I was trying to lift up on the foam tape on top of the cable (which lifted but of course didn't work).
2) Prying the old battery out was much harder than I expected. In fact, I broke one end of my spudger tool trying. To free the battery I used a wide flat blade screwdriver, and really had to dig under and use some force. When it freed up, it really popped, at which point I could work the screwdriver around and tug until it came free. You need to make sure you done get wires caught as you pry.
Other than those two things, all went smoothly. It would be simple to do again. Great product!