20 - 30 minutos
2+ year old iPhone 3GS kept shutting down unexpectedly. Determined by browsing web that problem was likely due to a worn-out battery. Ordered replacement battery from iFixit.
Battery replacement took ~45 minutes to complete. Instructions provided by iFixit were very good but, about half-way through, I began to get a little concerned that I would be able to get everything back together and working again. You really have to about totally take the phone apart to get at the battery. But, I kept going and, voila, after a little fiddling with one of the cables (see below) and some trouble getting the logic board precisely situated, the phone powered up and has been running well now for a couple days.
The only real problem I had was getting ribbon cable #3 off and back on again. The plastic tab, which secures that cable to the logic board, did not want to rotate up 90 degrees, as the directions said it should, but I was able to slide the cable out from under the tab and reinsert it without rotating the tab up. All in all, I am very happy with the repair and with the part and directions provided. Thanks iFixit!