5 - 30 minuten
Both phones running down in a day.
Both repairs successful. Phones hold a charge for several days now. Each replacement took about 20 minutes. Guides very useful but not 100% clear with respect to the little piece that makes contact with the back. The iPhone 4 guide called it by two different names. A few more pictures would have been nice.
Both phones came up searching for the carrier but a hard reset fixed that. No need to do a reset network (which loses your WiFi passwords).
Very small delicate parts and put on your reading glasses. Use fine point tweezers to handle things (not fingers). Go slow prying removing the battery connector (it lifts straight up- not to the side) and take your time prying out old batteries, the glue is pretty tenacious. A non metal tool like a chopstick or plastic screwdriver is a nice adjunct to the prying tool furnished with the kit. Study the arrangement before removing the internal screw(s). Have the charger ready and plug it in right away - these two batteries were at 74% and 22% but you never know what you will get.