2 - 4 horas
I have Mac mini with only 128GB SSD and empty 2nd bay. Time Machine backup stored on external USB HDD and used 1 usb port out of 4. As you know, USB ports are never be too many :) Previous minis has 5 ports and its covered all my external devices. Mid 2011 model have only 4 ports and useless SD card reader I never used. As you, dear reader, huh? :)
My thoughts was to use 2nd bay for Time Machine HDD and free USB port.
I have 2,5" HDD 500GB and googled iFixit dual hard drive kit with genuine Apple SATA cable and useful things to upgrade mini by myself. Guide was perfect and process takes only 30 minutes. Thanks iFixit!!
No advices, guide is smooth and clearly written. Just do it step by step. And yes - make backup before you start. Just in case :)