Most people have probably kept cheese lingering in the refrigerator a little longer than they should have, and had mold start to form. Cheese is a food like any other, and can't keep indefinitely. The shelf life varies from cheese to cheese, assuming you keep it refrigerated. Soft cheeses like ricotta or cottage cheese usually last for about a week. Semi-soft cheeses such as feta or mozzarella are good for two to three weeks, firm cheeses like Swiss and cheddar can last for five weeks or longer and hard cheese like Parmesan may survive 10 months.
When to Cut
While mold is not something you like to see on any food, there are times when simply cutting it off the outside keeps your cheese edible. If you encounter mold on firm cheeses like cheddar, Colby or Swiss, cut the moldy area off, plus about an inch on each side of the mold. Wrap the mold up and toss it in the garbage to avoid spreading it in your kitchen. Cut carefully when you are trimming away mold, because even your knife can spread mold spores if it comes in contact with the mold as you cut.
When to Discard
While it is OK to perform a little "surgery" and salvage firmer cheeses, soft cheese that has developed mold should be thrown out. Any cheese that has been shredded, sliced or crumbled that has mold, may have it all the way through and should be thrown away. Soft cheese like cottage cheese, cream cheese, Camembert, Brie or ricotta has the same issue and you should not try to salvage any of it if mold is present.
Dangers of Eating
Anytime you ingest mold, you are taking your chances, healthwise. Some mold spores may not cause you to become ill, but others that produce poisons called mycotoxins, certainly can. If you are unsure of how deeply the mold has penetrated, just throw it out. No piece of cheese is worth your health.