15 - 45 minutes
My previous battery was draining quite fast and thought a new battery would solve the issue.
When trying to unscrew the single Phillips screw securing the battery connector to the logic board, I thought I had stripped the screw but I just had to press a little harder when unscrewing and the screw came loose.
The old battery was glued down pretty tight and had to pull quite hard to peel the battery off the adhesive.
In summery, the process was smooth and easy.
Not really other than to press firmly when unscrewing the Phillips screw that secures the battery connector to the logic board and to pull firmly but too hard when pulling out the old battery or you will pull out the plastic tab.
I recommend that you get the liberation kit with your order, which will leave your iPhone 4 more repairable in the future. You are not charged extra for the kit.