2 - 4 horas
No real problem. I just wanted to add some speed to my Mac Mini, so I decided to put in a second hard drive.
Easy. Followed the iFixit guide and had no trouble. It was great to have every step pointed out with pics and clear instructions of what to do and what not to do.
No trouble on the install, but removing the power supply was a bit tricky. I had to get a pair of pliers to be able to rotate the AC-in connector. Actually putting it back in was relatively easy.