2 - 5 minutos
My iPhone was only two weeks old, and i paid a lot for it, so i had to fix it.
It did go really easy with the screwdrivers from the iPhone 4 Liberation Kit, but i didn't use the Philips screws, I just put the original screw back in.
If you want to fix the back cover of your iPhone4, just order it, it's really easy, anyone could do it. And you don't have to send it to somewhere else and pay a lot of money to get it fixed.