Not too hard at all - but beware the white screen.
20 - 30 minuten
The battery on my wife's old iPhone (3Gs) would not hold a charge and the phone had bloated pushing out the front display assembly and screen. It turned out it was the battery that had expanded. It was soft and bloated. I wanted the phone for my daughter to watch videos on Youtube.
The repair was fairly easy, but when I was done I got a white screen with no image. I thought I had done something wrong and thought it was because I had trouble replacing the ribbon cable attached to the screen/display assembly. I kept repeating this step over and over with no change. I finally read some of the repair questions and saw that a white screen may just require a reboot of the phone to fix. That did the trick.
I think the repair guide should mention the white screen problem and the easy fix so people do not spend undue time thinking they have done something wrong during the repair and replacement of the battery. Had this issue been mentioned as a possibility in the repair guide I would have saved a lot a time and effort. I must have removed and replaced the front display assembly a dozen or more times trying to fix the white screen (trying to insert the ribbon cable further into the slot) when a reboot of the phone was all that was needed. I had actually done it correctly the first time, but did not know that I had because of the white screen.