20 - 30 minuten
iPhone 3GS battery held little or no charge -of course it's 5 years old!
I replaced the battery on my wife and my own 3GS'. The first was mine in case I bricked the thing. It took about 35 minutes to watch the video and walk thought the steps.
Sadly, I was doing it next to my son who was having some intense fun playing minecraft at the time and at one point bursted out and I had to go in and dig out a screw I was aligning at the time with the included magnetic tipped screwdriver. It was towards the end, and I was frustrated at having to have to unscrew the previous work, since it seemed to be wedged in there...and focusing on that screw, the thin grounding wire snapped. A little make-shift tape job and it seems to be alright.
Oddly, being in the US, I still had that "in some cases of iPhone from outside the US have just a ribbon, rather than nice and neat connector..." situation. Lots of luck getting that pressed back in there, but I must have gotten enough in there, because both iPhone now work and hold a charge.
You left out that there is a tiny metal clip that holds the camera down which needs to be kept track of once you unscrew it's screw.
After any "be VERY careful not to..." statements, provide a link to the part or solution, for those that "were Not VERY careful." ;)
In battery replacement cases - I know this is a no-brainer, but when you have them reverse the process tout things back, remind them to plug the phone in and let it charge fully before trying to turn it on and thinking you've done something wrong because it doesn't come on - I just put a new battery in it, right? lol. It was late when I did the second phone and it dawned on me after going back in to check all the connectors.