10 - 30 minuten
I bought an old iPhone 4S in very good condition just to use as a basic cell phone and not pay the price of a brand new on. Since it's 4 years old, the original battery was not holding the charge well. Somehow, I found iFixit and ordered the kit (The battery store nearby wanted $40 for the battery and $50 to do the replacement job. Batteries from online stores have many bad reviews, so I was hesitant to buy cheap replacements.)
It was very easy to replace the battery by using the kit tools and following the step by step instructions, including detailed photos, in the website. After surveying the tools and the steps to take, the actual procedure took about 10 minutes, probably less. It was not difficult at all. The 4S is really easy to open up and to disconnect the old battery. After charging the new battery to 100% it held charge at maximum for a long time and now continues to live up to the original specifications of the iPhone 4S (specs in Apple's site and others).
Next time there's a need to replace parts for a mobile device, I will come to iFixit first.