10 - 30 minuten
My iPhone 4S is just over two years old, and the battery started to act a bit funky. I'm not a heavy user to begin with, but I would return home from work with the battery way below 50 percent. Also, I got a 20 percent warning from the phone, only to have the phone go dead within a minute of the warning. I had a couple of friends who had replaced batteries on their 4S phones, and they told me it was easy.
The step-by-step at iFixit was very clear. I did, nevertheless, seek out a video on YouTube to get another angle on the repair. It was easy to do. The only real trick was getting the antennae shield back in place when putting everything back together. The thing is tiny!
I had no problems. My advice is give your hands a good washing beforehand, to keep skin oil from contaminating the electrical connections. Use the plastic tool for everything you can, since you don't want to short out anything. The pull tab for the battery doesn't work as advertised, if you ask me. The battery is stuck in there good. Use the plastic tool to gently pry the battery up from the edge of the phone. Then use the pull tab. Finally, keep careful track of the two screws inside the phone, since they are of slightly different sizes.
It really isn't too difficult at all to replace the battery. Good luck!