10 - 30 minuten
One day, my wife's iPhone 4S was fine, the next morning, it was completely dead. The Apple Genius said the phone was shot, that changing the battery would not help. With six weeks before she was eligible for an upgrade, the "Genius" advised her to get a cheap phone and eke through until then. It seemed to me that the iFixit kit was inexpensive enough to make replacing the battery a worthy gamble - if the "Genius" was correct and a new battery was not going to help, I would only have spent $25.
The repair went really well - except for the part where I realized I ordered a battery for an iPhone 4, instead of a 4S. The connector is turned 180 degrees opposite for the 4, so I ran into a gumption trap there. I called iFixit and I swear the man who answered knew who I was and why I was calling before he said hello. It was uncanny. A new kit was sent out, the battery popped right in, screwed it back together and the phone was up and rocking again. Take that, "Genius!"
I would just say to make sure you pay attention to which screw is which when you unscrew the battery's connector. They are slightly different sizes and it's hard to discern them visually.
Also, on the top screw, just watch out for that black "contact clip," it's going to pop loose, so pay attention to how it is positioned before disassembly. A pair of needlenose pliers (or tweezers) is helpful here for positioning the clip for reassembly.