While hard cheese can stay out of the refrigerator and get warm without any safety issues, soft cheeses require more refrigeration. Cheeses such as ricotta and mozzarella should not be left out of refrigeration for more than two hours.
Whole Foods Market recommends that soft cheeses should be placed in a cooler with ice if they are going to be out of refrigeration for more than 30 minutes. Because soft cheeses contain more water, they provide a breeding ground for bacteria and are therefore more dangerous to leave unrefrigerated. Processed cheese, such as American cheese, can stay out of the refrigerator for longer than two hours without any danger.