20 - 30 minutos
The iPhone 3GS (2nd owner) was on its last legs and could not stay awake for long, so I ordered the replacement battery from ifixit in the hope it would contain the elixir of phone youth.
Followed the instructions, kept the various sized screws apart from each other, handled the old phone gently, and in 30 minutes it was raring to party. Very simple, and satisfying. The fact that the little philips screwdriver included in the ifixit kit is slightly magnetic was a big help.
Do it step-by-step on a clear surface. When you push the glass screen back in place (the final step), be careful that the rubber seal around the screen is placed correctly and does not get squeezed or cut; it is very thin and getting it in place without bits sticking out takes a few tries.