1 - 2 hours
The computer was just really slow - I was also installing a new 1TB SSD in my macbook pro so it made sense to use the parts that I had in the mac mini.
It took a little longer than I thought - stripping down every single part from the inside of the computer was not what I had in mind when I took the decision to make the change. Apple really does not want this done by someone at home and they make it really difficult. I could not have achieved it without the tool kit - there are so many speciality screws I would have not even got into the casing. That said, the repair was very successful and we are really enjoying the speed improvement at home!
be very careful about knowing which screws go where. When I put everything back together I had a few parts left over. Nothing rattles and I thought I successfully put everything back where it belongs. Photograph each section just before you take it apart for extra info.