10 - 30 minutes
Like many iPhone 4s owners, our 2011 phones are still shiny, fast, and competent. But they were showing the symptoms of ennui that a two-plus year old battery exhibits. (Fast "charging," sudden drops to no charge, and so on.) I ordered a pair of iPhone 4s replacement battery kits from iFixIt. I don't want to hear about how "awful" the USPS is, as I received the package just 3 days after ordering.
Being altruistic, I did the first replacement on my own iPhone. Opening was easy. The ground clip proved puzzling more than tricky, as it was called out only grudgingly in the Guide. (I submitted a modification to that step in the interest of clarity.)
Because I'm methodical, it took me about 30 minutes to do the first replacement. All worked flawlessly, including the surgical replacement of the Pentalobe screws.
The second iPhone, my spouse's, took less than 20 minutes to get its new battery installed. Everything seemed a little easier. Some of it was experience. But the ease with which I dislodged the old battery from its bed of adhesive was a bit odd. Then I remembered: my iPhone always rides along in my pants pocket, so it's been warm constantly (not just during calls) for the past two years. Hers is always in a handbag of some kind, and is more often cool or cold. Draw your own conclusion, engineers.
The troubles that people report with this repair often come down to "I can't loosen/tighten a screw." They often blame it on the drivers supplied with the IfixIt kit.
Here's the thing: you have to commit to those tools, because they're the right ones for the task. With small drivers and screw heads, it is important to insert the tip into the head firmly, and to apply some pressure to keep it seated as you turn.
When inserting a screw, get the shaft placed, then give it a short *counterclockwise* turn to line up the threads before you start rotating clockwise to turn it in.
Follow these two guidelines, and your success rate will increase by, oh, say, 82 percent.