I like to buy in bulk because I will never forget this route. Every time I change my bag I write the date on the new one so I know when to change it. (usually after three weeks). This bag catches dirt. Living in the country with a dog, a dusty road and daily activities (but we take off our shoes when entering the house except for the dog), I can't tell if the vacuum is the reason that the bag is so full or if it is the bag itself. But whatever it is, these are great wind tunnel bags.