2 - 4 horas
Wanted to add a 1 TB harddrive to my MacMini late 2012 running EyeTV media center and ordered the iFixit Mac Mini Dual Harddrive kit which arrived promptly.
By following the super instruction on the iFixit website I had the drive successfully installed in less than 30 minutes. It has now been running superbly for over a month ;-)
The only advice I can give is that you follow the iFixit instruction carefully, then you should have no problems installing.
PS! I am turning 70 this year (2013) ;-)