The guidelines issued by the USDA state butter can be left out covered & unrefrigerated for a week.
Butter storage life on the counter or in the refrigerator is shortened by light and circulating air. Have you ever noticed when butter is left uncovered it picks up the flavor of food in the refrigerator or odors in the room? The biēm helps in both of those ways by eliminating light exposure and circulating air, once the cap is sealed.
That being said, locations and room temperatures for storage and use vary. We recommend leaving the biem out while in use during the day and storing in the refrigerator overnight or simply storing in the refrigerator when not in use (with the butter still inside the biēm). Storage times can be different depending on the temperature of where you store the biēm but we do not recommend storing in direct sunlight. If you forget to refrigerate overnight, it won't be a problem, but if you store it in a warm place and kept it out for a few weeks, the butter might need to be replaced and the unit cleaned.