Butter storage lifetime
We store all butter other that what's currently in use in the freezer, where the shelf life closely approximates "forever." Can't recall ever having a bad or rancid stick from the freezer. Many folks in moderate climates keep the currently in use butter on the counter, not in the fridge (spreads easy, spoils a bit faster, don't try in hot climates.)

Depending what you are using your butter for, you can also increase effective storage life considerably by making it into clarified butter (or ghee, same thing, different names in different parts of the world.)