20 - 30 Minuten
The battery on my Wife's 3GS would barely last a 12 hours with minimal usage so it was time to change the battery.
The repair went good. It took about 30 - 45 mins to do the repair. The included tools worked for everything I needed. I used a suction cup from my radar detector. The new battery is working good and the phone is back to original life.
Getting the sim tray aligned is a pita. Take your time and when it's right the cam will pop into place like it should. Also, after replacing the battery plug in the phone to make sure it'll charge and let it charge to 100% before powering it on.