20 - 30 Minuten
After the last OS update from Apple my 3GS would repeatedly crash. Most often I could not get it to restart until it was plugged in to the outlet. It would not crash as long as it was connected. After searching the internet I found a post on the Apple forums with many people having this problem. Most users were able to resolve it by replacing the battery. Ifixit was suggested there.
The step-by-step instructions with photos were great. My phone has not crashed since.
It is not as simple as removing the case and slipping in the new battery. You do have to completely disassemble the phone so go over the directions and make sure you are comfortable with that first.