20 - 30 minutos
Before I replaced my battery, my phone's battery life would be down to 70% within about an hour of unplugging the phone from the charger, and 50% after two hours. Usually around 50% the phone would just shut off and the back of the phone was very warm. I'm waiting for the iphone 5 so wanted to try to get the most out of my 3gs as possible.
Battery arrived quickly. Took two or three days, which was a day earlier than the originally estimated delivery date. The first time I took apart the phone, it took about 30 minutes to get it apart and back together. Note: the directions are good but be careful getting the connector off that has the flip up part - it doesn't flip up the way you might think and it's easy to damage the flip piece if you aren't careful! Aside from that the directions were great. After I replaced the battery, I turned on the phone and it immediately would power up, after about two minutes shut down, reboot, and power up again. Took everything apart and put back together in case i'd missed something. Same behavior after I got it all back together (btw it took about 15 minutes the second time). I also noticed that when i plugged in the charger, it would charge for about a minute, then show the "plugged in" icon, then would shut off. Appeared to be a problem with the charging circuit in the battery. Took everything back apart again, and put in the original battery. (3rd time took 10 minutes) Phone stopped doing the reboot thing. Of course back to bad battery life but at least i had a phone. Emailed ifixit and on Monday got a response that they were sending a replacement battery, no need to send back the original. Two days later the battery arrived. Installed it, and it's been working perfectly! From a full charge, it goes all day without needing a charge. I don't think that the capacity is as good as the original battery when I first got the 3GS (though the software has gotten complex enough that maybe a day without charging is good?) but i'm happy i don't have to keep the phone plugged in constantly now. I feel i'll get at least another six months out of the phone or however long it takes to get the next generation iphone!
Definitely get the suction cup to get the screen open. I used packaging tape which worked, but it's a little difficult to get the tape at the right angle to get the screen to pop up. If you have a screen protector on your phone, may as well order a new one because you'll probably destroy the old one in the process of taking the phone apart. Be careful with the connectors.