20 - 30 minutos
I purchased a refurbished iPhone 3GS online at my favorite computer retailer as a Christmas present for my girlfriend. Got it, charged it up, and within an couple of hours the battery had lost 20% and it had been completely turned off!! Rather than go through the hassle of returning it, I took a chance and bought a new battery.
Repair went very well, the instructions were great. Would have liked to see a little more detail for the re-assembly, and not just "follow the instructions in reverse". The phone actually lasts for a whole day now. :-)
Lock your cats out of the room before you attempt this. The screws are VERY SMALL and hard to find when they're knocked on to the floor.