20 - 30 Minuten
My son's 3GS wouldn't even last 24 hours with the screen brightness at 0, airplane mode on, and sending about 4 text messages a day with almost no cell usage - time to replace the battery.
Replacing the battery is pretty straight forward. I'd already replaced other iphone parts, so this repair took me about 20 minutes. Easy to follow guide.
Be careful when you peel away the existing battery from the case - don't leverage something against the case or it may crack. Didn't happen to me, but I could see how it might happen if you didn't pay attention. Use the ribbon connector with the screw hole to line up the battery when inserting the new one, you want to make sure it reaches the "pin".