5 - 30 minutos
I was replacing the battery on my iPhone 4, because mine was getting shorter and shorter battery life.
The guide was very good except it didn't warn how well the glue Apple uses can be
There is a small contact that comes up when the battery connector is unscrewed, just make sure to keep note of how it goes in for when you're done.
I ended up stretching the tab almost an inch before I decided I needed to use a spudger to get the glue to free. Once I did that, the battery came right out, new one dropped right in.