20 - 30 minutos
The old battery of my iphone 3gs got thick so that the back panel got cracks and the display was pushed out of the case. At that point i couldn't use it only while having it plugged in. Even then sometimes it just shut down.
The repair was very easy! I just had to replace the battery. A few steps and it was done. The only tricky part, where i really had to pay attention was when pushing the ribbon cable back into the socket.
The ribbon cable has to be as far inside in the socket as possible, but you've already reached this point when you can't see the shiny parts of the connectors.