5 - 30 minutos
2.5 year old iPhone 4 battery was starting to hold less charge. The phone works great otherwise so no need to buy a new one.
Repair was very straightforward. Took about 10 minute total following the step by step guide on the iFixit site. The 10 minutes include the time it took to explain each step to my 3 year old son who wanted to help me replace the battery.
Take it nice and slow. The plastic pry tools are a must have. The old battery was very sticky so the pry tool helped removed them. Also work on a big clean surface. The screws are tiny and if you drop them on carpet, they would be nearly impossible to find.