5 - 30 minuten
My wife's iPhone 4 battery was running down very quickly.
Smoothly, with two exceptions: 1. A very small part fell off the phone when I removed the two screws holding the old battery connector in place. iFixit's video didn't emphasize this very much; the woman on it only mentioned the 'antenna connector' briefly in passing. I couldn't tell for sure where the part went, or if it was important.
2. I now realize the original iPhone 4 battery connector had 1 tiny screw holding it in place. The replacement battery has what look like 2 holes for screws. This is why I removed the 2 screws on the phone...one of them held the antenna connector, which fell off.
I managed to figure out where it appeared the antenna connector was supposed to attach, and screwed everything down. Fortunately my wife's phone seems to be working fine, so everything turned out well.
The whole process took about 45 minutes or so, with a bit of time being used to deal with the antenna connector issue.
I'd suggest that iFixit point out that the configuration of the battery connector attachment plate differs from the Apple OEM battery (2 screw holes vs 1 for OEM). It might also be a good idea to mention the antenna connector and what function it serves.
Unfortunately, my wife needed her phone back in service ASAP, so I was unable to take photos to illustrate the issue.