20 - 30 minutos
I wanted my parents to have an iPhone. A new one was pricey. My buddy had a 32 GB iPhone 3GS that was in great shape ... but was over 2 years old and the battery was really giving diminishing returns.
It took me about 2.5 hours mostly because I was very, very cautious. The documentation on the website was all that I needed. It was thorough and helpful. Glad I did it and I'd use iFixIt again. The 3GS is in great shape and now has a new battery.
The one thing I would recommend is to try inserting the SIM tray before you screw the logic board back down. In my case, the getting the camera to stay in its hole was just difficult enough that it pushed up on the logic board so that the SIM slot was not properly aligned with the case. I put the phone back together, discovered it was misaligned, and had to disassemble the phone.