20 - 30 minutos
Recently my iPhone 3GS began shutting itself off randomly during use (i.e. internet, checking mail, etc), even though the battery would show plenty of charge left. What made the problem worse is that once the phone shut itself off, it would not turn back on without being plugged in, which made it very unreliable. The problem became worse and worse, so I changed out the battery.
After watching the instructional video on ifixit.com (multiple times) and following the step-by-step instructions, I successfully replaced the battery very easily. It took me about an hour, but I was trying to be very careful not to break or rip anything.
Just like in the video, the spudger and suction cup came in very handy (without the latter, I'm not sure I would have been able to get the phone open) and made the process hassle-free. The magnetic screw driver that came with the replacement battery was very helpful, as the screws are extremely small. Also, I made sure to keep track of each screw carefully, as they were very easy to lose. The video and the step-by-step instructions made this repair a breeze, thanks ifixit!