If your butter tastes stale, bitter, and has a strong smell, it’s probably rancid. Rancidity is the result of the fat oxidizing. This process is accelerated by exposure to light, heat, and contact with certain metals (for instance, in utensils).
Butter usually can be kept out for several days without going rancid (salted butter will keep longer because salt acts as a preservative). Exactly how long butter will stay fresh at room temperature depends on how much heat and light it gets and whether it’s wrapped. Ceramic butter crocks or “bells” extend butter’s life span because they keep it cool and protected. Chowhounds have discussed different butter bells.
That said, the California Milk Advisory Board recommends keeping butter wrapped and stored in the coldest part of the refrigerator for optimal freshness and flavor, and to keep it from picking up unwanted odors. Butter producer Land O’Lakes advises against storing butter in the butter keeper on the fridge door, as the temperature there may be higher than elsewhere in your refrigerator.
In a typical fridge, butter will keep for as long as four months. It can also be frozen for up to a year (longer freezing may impair flavor and texture).
Room temperature, refrigerated, and frozen butter all should be stored tightly wrapped or in a covered dish, advises Emily Luchetti, executive pastry chef at San Francisco’s Farallon restaurant. “Butter can pick up so many flavors, regardless of whether it’s in the fridge or not,” Luchetti says. “Even if you leave it out, it’s best to cover it with aluminum foil.”