Bed pillows, on average, should be changed every year if you sleep on them nightly or every two years if you sleep on them periodically. Over time, they can become a breeding ground for dust mites, creating an unhealthy sleep environment in your bedroom. But what should you do with your old pillows? Use the old filling to freshen the bed your dog sleeps on. Simply wash the pillows and tumble dry them in the dryer. Rip open the pillow and replace the old fill with the new in the dog bed.