1 - 2 hours
I am using Mac Mini's in a video playback role. I have three that I am setting up for this purpose - two were servers with a second hard drive already, one was a standard single drive unit. I was placing a SSD drive into each as the second drive/Data drives.
The first one took the longest just because I was following the PDF very closely. I would not call this a difficult installation at all - just pay attention to details of the small cables that need to be disconnected/reconnected. First one may have taken 45 minutes, the last was maybe 15.
Make sure you find the locating pins and matching indents when you replace the primary drive. I found it useful to put the Mac Mini vertically on edge to engage these easier.