20 - 30 Minuten
The battery on my wife's 2-year-old iPhone 3GS would not hold a normal charge. It had to be charged every day.
I followed the instructions very carefully, so it took over 2 hours. I had to use a hair dryer to soften the adhesive holding the old battery on. Because I was VERY careful, it all went well.
This is too hard for the ordinary person. It was an order of magnitude harder than working on a laptop, which is an order of magnitude harder than working on a desktop. The hardest part is getting the cables back in place. There is a distinct snap that you can feel but not hear when they go back in place. But I am very grateful that iFixit had the parts and instructions available, because it worked for me. The only downside for me is that, rather than being grateful, my wife says I was too cheap t buy her a new phone! BTW, I have a PhD in experimental physics from Harvard.