20 - 30 minutes
I have an old iPhone 3 that I use on wifi to stream music in my garage. It was no longer booting because it wouldn't charge at all.
I got the battery and all the tools I needed from iFixit. I didn't realize until I started to take it apart that the case looked like it was bulging. When I opened the case (the suction cup worked great) I could see the main circuit board was bulging up and the screws literally popped out as they were loosened. I figured the main board was toast. I followed the iFixit directions and the new battery went in without any problem. iPhone works great again.
Take your time and follow the instructions very carefully. There are subtle differences in the (extremely tiny) screws that are nicely pointed out in the online instructions. Made reassembly a breeze. Also be very careful with the ribbon connectors on older phones. Mine were somewhat brittle.