2 - 4 horas
After having a SSD to boot from in my MacBook Pro, performance on my new Mini was feeling pretty sluggish.
Everything went smoothly with the upgrade (also took the opportunity to upgrade to 8 gigs of ram) and my old drive is cloning to the SSD now. Took about 40 minutes total because I ran into a bit of trouble aligning the top hard drive when putting everything back together. Everything looked fine, but the antenna plate (which screws into the top hard drive) wasn't fitting back on quite right. Just took some fiddling and all is good now.
I think I might have been running into trouble with the power cable going to the logic board. The top hard drive was being pushed up/to the side by it, so make sure that it is sitting nicely in the space next to the lower hard drive when reassembling.