The frequency of changing the vacuum bag will largely vary depending on how much you use it, the capacity of the bag, floor type, pets at home and others. Generally, a box of the bag should last you a year or so. Smaller bags need cleaning once in a month while larger ones can hold dust for more than two months. Cleaning pet hair and vacuuming carpet hair are two things can fill up a bag faster. To solve these problems some brands have included bag replacement alerts. As the bag start to fill up the airflow from the vacuums starts to reduce, these indicator senses the low airflow and tells you when the bags need a replacement.