10 - 30 minutes
My wife's iPhone 4s battery life was decreasing, so I decided to replace the battery in hers and mine.
The iFixIt instructions were clear and correct. The liberation kit (included with the battery, thanks!!!) contained everything I needed. The only minor hitch was that I hadn't noticed where the small ground clip went. The photo for this step could be enhanced a bit to highlight the part. However, in the end I believe I did reinstall it correctly.
The instructions state that you should clean the interior as grease/oils from your hands can cause wireless interference. I decided to avoid the issue by wearing rubber gloves (the thin kind used in doctor's offices) while changing the battery.