2 - 4 horas
HDD that comes with Mac Mini is quite slow, already used to performance boost offered by SSD I decided to upgrade my Mac Mini, but I needed traditional HDD capacity too.
Relatively easy, all in all it took about fifteen or so minutes. Since I tend to carry it with me to office - checked so that both HDD and SSD sit tightly in case and don't jump around, nice.
Take your time, don't rush it, before taking next step take a closer look to ensure you have not forgot any cable. Just have to be careful and patient - connectors are tiny.