Peanuts - how long will they last?

Although my peanut feeder is proving popular, the birds must only be taking tiny amounts as the level of peanuts hasn't yet visibly diminished. How long can I keep the peanuts in there before they go bad and need replacing?

Also, I just read on another site that you should move feeders regularly to avoid a buildup of bird droppings in one place - is this the case? I am limited as to the number of places I can put mine!