20 - 30 minutos
My iPhone 3GS's battery was no longer lasting through a day at work and it was looking like time to swap it for a new one. Also, I had another 3GS which had been in a pocket when falling off a boat into the ocean.
To prime myself on the dis-assembly, I decided to take a crack at the formerly water logged 3GS. With it apart I realized there was some minor corrosion which was easily cleaned up, but the glaring problem appeared to be the battery, which had expanded and was obviously effected by the saltwater. I then took apart the main 3GS, popped out the battery, swapped in the new one, then installed the older but still working battery in the phone that took a swim. Assembly went smoothly on both.
Popped both phones in the charger, and both of them lit up after a few minutes. $25 and I've now got at least 1.5x the iPhone at my disposal.