30 minutes - 2 hours
This Mac Mini I bought through ebay last year, but the guy that put it on ebay - had not upgraded anything on it (except for upgrading the OS to OS X Snow Leopard)
I installed a 1TB drive not once, not twice but three times - thinking that the computer was not recognizing it (dummy here ((me)) forgot to create a partition for the computer to write the information on first). Except for that little detail, the installation went beautifully - except for the bumps I kind of expected while putting it back together.
Six words - "Plan it out step by step". Because no matter how smart you think you are, you're still only human - you can still forget something along the way or just miss something due to oversight.