20 - 30 minutos
While on our road trip my wife's 3GS decided to not boot after the battery drained and we charged it! When we got home I tried to force a restore on the phone with absolutely no luck and many iTunes errors resulting. Doing a good bit of looking online I came across just a couple instances of people having similar issues and was caused by a bad battery. I was skeptical to say the least, but, my wife is horrible about syncing her phone and was about to lose half a years worth of pictures and videos (including a couple of weddings) if this wasn't fixed.
The repair itself was extremely smooth and easy, especially with the guides available on this site. I was able to get the whole repair done in less than twenty minutes (not my first iPhone teardown, lol)
While I found the repair to be very easy, I could see how this repair would be difficult for the heavy handed and impatient. You have to remember you're dealing with very fragile and precision cabling. You have to follow the instructions carefully and take your time with the cables/connectors to be sure not to break them.