2 - 4 horas
My mini was lagging when the hard drive was taxed.
The repair is a tad hair-raising. I've built gamer systems like forever, and swapped harddrives in a few dozen laptops. This is something else though; it's obvious that Apple did not really intend this to be a user-installable part.
It took me about an hour, and there was one deviation in my mini to the one used in the guide: In step 3 there's an extra screw holding the fan, which turns out to be the yellow screw from step 14; it's simply longer and holds the fan as well.
Other than that, the guide was well written, and not too hard to follow.
One complaint: The tools sent in the kit are sub-par; I cannot recommend that they be used. Get something where the bits fit the screws better.
Get a better set of tools for this. Any decent diy shop will be able to help. Don't skimp on those; the money you save will not pay for a new mini ..