5 - 30 minutos
After almost 2 years of usage, the battery in my iphone 4 was requiring a charge more than once a day. Getting sick of it, I just ordered a battery
I just followed the instructions on the ifixit website, along with their video, and it was done in less than 10 minutes. My thanks to the staff who wrote up the instructions.
Just read the instructions before you tear into the device.