20 - 30 minutos
I used my silly 3GS so much it's battery life became miserable.
The first battery that iFixit sent wouldn't charge. No problem! They sent me a new one and I didn't even have to mail the broken one back. Between switching back and forth between the new battery and the old battery and finally getting a working new battery in place I performed the iPhone 3GS battery swap about 10 times - we are now talking expert city.
Some of the connectors are a little hard to get aligned when clicking them back into place. This is very dangerous and pushing too hard could bend pins. Just be patient and make sure you have good lighting so you can look at the phone from several angles to get the connectors aligned.
Connector #3 and its ZIF socket can be intimidating, but it is actually one of the easiest to get right. The hardest part is pushing the cables from connectors #1 and #2 out of the way so you can get a grip on the right ribbon.
Putting the circuit board back into place requires moving Cable #4 and #7 out of the way. Keep an eye on them!