5 - 30 minutos
My original battery was almost dead. Less than a day to drain it completely.
Decided to replace it with a new one.
Nothing much easier than a batty replacement on an iPhone 4!
I used a surgical gloves, avoiding to touch the internal components with my nude hands.
An advise: befor start to replace the battery, you should first wash your hands with gloves weared, in order to eliminate the powder (or talc, don't know how you say in English :/) over the gloves.
Hope I'm clear in this explenation :)