Soft cheeses begin to grow mold within one week of being opened. Semi-soft cheeses, such as mozzarella, Monterey Jack and feta, last about two to three weeks without mold. Firm cheeses, such as Cheddar, Colby and Swiss, can last up to five weeks.
The cheeses most resilient against mold are hard cheeses, such as Parmesan, which last up to 10 months. The key factors that influence the growth of mold are the pH level and the water activity. Some cheeses, such as blue cheese, already contain mold when they are consumed. Some mold on cheese can be toxic, so always double check before consuming cheese that contains mold.