20 - 30 minutos
The battery of my iPhone 3GS didn't hold the charge anymore, and my phone would cut off every now and then. It was crying for a new battery.
The repair guide is neat, and following it step by step helped me out in a breeze !
Be sure to keep track of the tiny screws of different kinds. I made a rough drawing of their emplacement, and placed the screws on it, so I wouldn't lose them, or put them back at the wrong place.