10 - 30 minutos
The battery is my wife's iPhone was dying after a year of use as most Apple batteries go (charging twice in a day).
Using iFixit's guide on how to replace the iPhone 4S battery, the repair was simple and painless. I was done in 15 minutes.
Try to save the adhesive from the old battery. Because the new battery doesn't come with new adhesive, the new battery could roll around a little after installation without that adhesive.