20 - 30 Minuten
My iPhone 3GS was not functioning correctly for quite a while. Each time it was unplugged, it would work for a random amount of time until it shuts down for no reason, leaving a faded ghost image of the last screen for a second or so, before shutting down completely.
I had planned a whole Saturday morning to do it, considering my cluelessness of electronics and thinking I'll have to label the inner cables with small numbered stickers as shown in the videos and photos. So my surprise was huge when I cracked open the iPhone and saw that the stickers are built-in! Thank you Apple (but then again, if they think of a possible battery replacement, why don't they make it simpler?..)
Then I proceeded to the steps shown in the pics and vids on ifixit.com and to my great surprise, I've done the job in a quarter of an hour. i didn't believe I had already finished and thought for sure that I screwed it completely. That has now been two weeks since my DIY repair and I can say without a doubt that it gave my iPhone a second life: everything is in order, and some functions improved (my camera is cleaner since this fix, as I removed some inner dust, and my buttons are more responsive). So thanks a lot guys for your wonderful site and easy step by step guide!
Just do it! There's nothing complicated and if I've been able to do it in 15 minutes, the average human being can complete the task perfectly in ten or less!!! Crack your iPhones open and give them a second life without the ludicrously expensive Apple tech support!