2 - 4 hours
Wanted faster performance on my base model Mac Mini. Installed 8GB of RAM, used second drive kit to install a sata3 SSD into the open slot.
The tear down to get into the empty top space for the new drive was easy. Putting it all back together was a bit more difficult, mostly because the fit on the bottom drive needs to be precise and it is impossible to see if you have the mounting screws for the bottom drive in the holes on the inside of the case.
Use the antenna plate to help you align the bottom drive. Attach the two 6.6mm T8 screws circled in red on step 8 of the ifixit install guide to attach the drive to the plate prior to pushing the logic board all the way in. Attach the other connectors (airport, IR, drive connectors and power) and use the fit lobes on the sides of the airport plate to guide the drive into place. This made getting the bottom drive back in and seated correctly a snap.