20 - 30 minutos
My iPhone 3GS battery life got progressively worse over a period of months, crashing constantly at 50, 60, 70,80%, until eventually it would shut off still showing a 90% charge. I decided to try fixing it myself for $30 rather than spend $70 to ship it to Apple & be without my phone for a week.
Following both the iFixit repair guide and a few related YouTube videos that did a step-by-step repair, I had no serious problems replacing the battery & now have a working device that's as good as new. Also happy I opted for the iFixit kit instead of cheaper alternatives that seem to fail after a few charges. This one works great so far.
The guides aren't terribly clear on removing/replacing the logic board. The various connectors kept flipping down and getting stuck underneath, and required a bit more attention than expected to get them into place w/o damage. It's also important to ensure that the SIM card fits in easily after replacing the logic board but before doing anything else. You'd probably be able to reconnect everything otherwise, but then couldn't insert your SIM. Mine eventually clicked into place after a bit of fidgeting w/it & all works well.