2 - 4 hours
Hard disks are expensive these days and I had a spare SSD. I wanted to replace my PowerMac G4 which held the time machine backups and I needed an iTunes server for the Apple TV and to sync the iDevices wirelessly. So I took the cheapest mini which is like 100 times faster than the G4 I guess and installed the RAM and Disks I have.
I wasn't very comfortable doing this, I didn't want to mess up my shiny mini, but it all went good. I was astonished at how they managed to cram in those parts while keeping it relatively simple to maintain.
It's VERY important to insert the upper hard drive very precisely, otherwise you can't install the antenna plate. The antenna cable is very fragile, don't insert it before you got the drive right!