How long after purchase does hard cheese keep?

It depends on the  moisture content of the cheese. Very hard cheeses, such Parmesan and Romano, can keep for up to a year. Semi-hard cheeses such as cheddar, Swiss, Colby and Gouda ,will keep for about 1-2 months in the refrigerator if the package isn't opened, 3-4 weeks if opened, and 2 weeks if sliced.  In general, the less moisture a cheese contains, the harder it is and the longer it keeps.
 
Source(s):
The Cook's Thesaurus "Semi-Firm Cheeses"
Ehow.com "When Does Cheese Spoil?"