10 - 30 minuten
My battery was about 2 years and a few months old. It didn't do its job properly anymore - still guessing whether it's just an old battery or is it a software problem (and my oldie can't handle it anymore). Had to charge it 2 or 3 times daily. And, no, I don't consider myself such a heavy user. After all I have my computer before me most of the day, and rather use it, instead of staring into iPhone's tiny screen.
Even the new battery now... I charge it once per day and I do kill all apps once I'm done with them, and don't use BT (but do use WiFi and GPS - though no significant difference with those two turned off). So, yeah, it's probably that the hardware is getting too old for the new software (iOS8 is coming in September/October, then 4S users are done anyway).
Didn't go as smooth(ly) as expected. One of the screws on the battery was stripped. Not to mention it was a bit stuck as well. What made me wonder was, how could it happen? I got a new phone out of the (wrapped) box, and yet one of the screws wasn't okay. In the end I decided to go with the theory - probably some Apple's policy (so that I'd rather take my phone over there and pay 200$ for a replacement - nooooo, thank you). Finally after an hour or so it was done. Once the battery was out it was like a walk in a park.
Order an additional set of screws. Just in case. I had to close my phone (cause the first time there was nothing I could do) and reopen it, when the additional tools arrived. And by then I was two screws short, not to mention the one on the battery with stripped head.