1 - 2 hours
I had a new late 2012 Mac Mini and a 128 GB SSD that I pulled from my old Macbook. I ordered the dual drive kit to take advantage of the SSD and make my own Fusion drive.
Worked pretty well. It is kinda gnarly going inside of the Mac Mini, but it all worked out well. I messed up the first hard drive cable, but iFixit sent me a free replacement (even though it was totally my fault) so I was able to get the job done. My hard drive is now way faster than the 1 GB that came with the computer and boot times and app startup times are greatly reduced.
Be super careful, if you have not taken apart a Mac before, this is not the done to start with. I had to pull the mother board back out because I forgot something and failed to remove the hard drive cable, fortunately it was the cable that snapped in half, it could easily have damaged the motherboard. Oh, and Macsales.com has good instructions for setting up the fusion drive, it takes using the command line version of Disk Utility.