20 - 30 minutos
A friend of mine had an iPhone 3Gs that was shutting off when the battery read 70%. It would turn back on within 3 minutes with a hard reboot after it was plugged in, but would read 40% charge. In other words, we could get through half of the Star Trek Movie and then have to take a short intermission, which is great if you've been drinking a lot of soda, but not so great if you really like the movie.
After an exhaustive internet search (a very long 10 mins) we discovered that the battery was the culprit.
It was the easiest thing I have done in a long time.
USE IFIXIT's GUIDES...THEY ARE WORTH THIER WEIGHT IN GOLD (and that's saying something these days).
The only thing that wasn't satisfying about this project was that we never quite figured out where the missing 30% of the battery went.