10 - 30 dakika
My iPhone 4s is only 18 months old, but the battery was only lasting about two hours without a charge. Thought about upgrading to a 5s, but that would be $650 for the same memory (I have the 64 GB model 4s). So I decided to go for a new battery.
After reading through the instructions very carefully, and making a few modifications to the instructions after reading the comments, my repair was flawless.
Make sure to loosen the old battery BEFORE unclipping it from the circuit board. That way you don't loose the grounding clip while trying to get the battery out.