5 - 30 minuten
I got my iP4 on release day. I'm a very heavy user and battery wasn't what it used to be after a year.
This was a very easy fix. Only took about 10 min with the right tools from this site.
Watch the videos. Carefully remove the battery, it has some glue under it. Prying around all edges before pulling the ribbon helped me. Also the connector on the new battery had to be bent a bit for it to snap on flush like the old one.