20 - 30 minutos
My iphone 3gs was constantly shutting down, and the battery was not dead. At first, I thought it was a glitch with the upgrade, so I did a system restore. I issue was still there after the restore. My phone would work fine, as long as it was plugged either to the computer or the outlet. So I decided to try the battery, and now it's working flawless.
Since I've replaced my touchscreen and digitizer in the past, I knew how to open the phone, and remove that portion. However, I never removed the circuit board. This was fairly easy, as long as I take my time. The hardest part was removing the old battery, because of the glue. I actually broke my prying tool, because I was trying to use it to remove the battery. After that, I used a set of pliers, and pulled the tab of the battery, and it came off without any problems. Put in the new battery, and installed everything in reverse.
If your not good working with your hands, please have some else do this. It's fairly easy, you just have to take you time, because of all the small components inside your phone.