Battery dying? End of the world tragic to a teenager...
10 - 30 minuten
My daughter has had an iPhone 4s for about 16 months. Being a teenager, it is never off, and never not being used. I replaced the screen protector a couple months ago and found that it had actually melted onto the screen from overheating, but that was easily cleaned and not the issue here... The issue is that these batteries do not last long if you use the phone on a regular basis, let alone over-use it like a normal teenager. By the time I got the kit from iFixit, the phone would reboot randomly and the percentage meter would perform tricks that David Copperfield won't do.
The repair went smoothly. I had gone over the directions multiple times before the attempt, and had checked a number of other stories to be on the watch for any gotchas.
End result - 10 minutes and no gotchas. The phone booted first try and found the signal in the normal amount of time it took before the replacement. As far as I've heard, the phone hasn't had any issues since and the battery appears to be lasting (and behaving) much better than before. The review from the kid was "I haven't been paying attention to the battery", but since I was getting "This battery sucks!" on an hourly basis before the replacement I think there is measurable improvement.
I have no problems working with hardware, so this was completely stress-free for me. If you're worried, there's no need... pay attention to the instructions and any possible gotchas and just take it easy - this replacement will go fine.