5 - 10 minutes
Frustration often leads, indirectly, to broken things, such as cracked smarphones.
It took roughly 30 minutes to fix. The S4 is easy to take apart aside from separating adhesives. I spent some time prying the glass off which caused little fragments to spray everywhere. This was completely unnecessary. Eventually I got under the display itself and it came off easily. Installing the new display is a snap.
I was hesitant since this repair supposedly involves the use of a heat gun, but there's a good youtube video for this exact repair (entitled "Galaxy S4 Display Replacement | Removal from Midframe") which explains that the screen shouldn't get hotter than 100F. I used an IR temperature gun to make sure I just barely warmed the phone. The verbal description of the repair process was a bit cryptic but did include important details, such as going under the foil behind the display. These phones are made of many layers and it's important to know which layers are being split apart. The layers are not all the same size or shape, either, so pay attention to the phone's design.