20 - 30 minutos
The battery of the iPhone 3GS of my wife did have a low "performance"
Well, all the process was quite laborius: a lot of screws, cables and little connectors, but with the help of the iFixit manual I was able to reach the battery, remove it and replace it with the new one.
In my case, the more critical step was reinsert the #3 connector. Be careful with it