I was procrastinating - but all went very well!
2 - 4 horas
After 20+ years with Windows I decided to finally migrate completely to an OSX environment and bought a mid-2011 Mac Mini with 8GB and the standard 5400RPM hard drive. Much to my dismay the hard drive was a real bottleneck and ordered the iFixit dual HD kit with tools and a 256GB Samsung 830 Series SSD
I am relatively technically experienced but taking apart a brand new Mac, invalidating any warranty made me procrastinate for some time. Read up A LOT on the MacRumours-forum, watching videos on YouTube and building up confidence.
Naturally I went for the iFixit instructional video, cleared the table and ensured there was sufficient lighting available and went to work
Many of the things you will be asked to tinker with are very very small, so my advice is to ensure there's appropriate lighting available.
Place screws and various small parts on a white cloth or tray, ensuring you don't loose anything.
At times it might feel like mild force is necessary, for instance when placing the power supply back in the chassis but give it time and get to know the chassis before you proceed with too much brute force.
At the stage when the additional disk has been placed in the lower dock, your Mac Mini will be in atoms. Once you're ready to rebuild your Mac Mini you'll find that the iFixit video doesn't cover this but that is not such a big deal once you look at the parts, just backtrack in the dismantling video and you'll be fine =)