5 - 30 Minuten
Battery life in my iPhone 4 had deteriorated from a full day or more to 2-3 hours.
Replacing the battery in my iPhone 4 was every bit as easy as advertized, the job took me just 20 minutes start to finish.
The caution in the instruction video about excessive glue under the battery was good, so just go slowly and gently pry up around the perimeter, it'll come loose. Battery life for the first several days was not very good, but after about four days battery life was almost like new. My guess is that you need give it a few full cycle of charge and discharge before you get full performance.