20 - 30 minutos
My iPhone 3GS, which I've had since the day it came out, was experiencing intermittent shutdowns. Irregardless of the displayed battery percentage, my phone would often heat up and just randomly shut off and die. The only way to reboot was to plug the phone back in. I needed a new battery, so I decided to try iFixit's self-repair kit.
Very smooth. Performed the battery swap in under 30 mins, being very careful and double-checking each instruction as I performed them. Along with the battery kit itself, I ordered the spudger and suction cup, both of which were extremely useful during the process.
Be patient when trying to pry the old battery off the back of the iPhone case. It's attached with a small amount of adhesive, and was tricky to get off. Get a little bit of leverage off of the edge of the case and use the spudger to reach deep underneath the old battery to remove the adhesive.