1 - 2 horas
I wanted the benefits of an SSD as well as the 1tb drive that came with my Mac mini but didn't fancy paying Apple's high premium for upgrades so I bought a 256gb SSD from crucial and this upgrade kit from ifixit.
Very simple. I followed the instructions and despite having to completely dismantle the computer it all went pretty smoothly.
My tip: pay attention to the orientation of the rubber surrounding the power input, it's not obvious! It took a few tries before it all snapped back into place because of that.
Also: follow the written guide as well as the video. The video is good but the written guide is more detailed.