Granted you invest in a vacuum that has a bag installed in it, you may be wondering how often you are going to need to replace it. It sure would be nice to provide you with a definitive answer to this question but with so many variables at play, that simply is not possible. For one, the model of the vacuum will make a difference but so will how you use it. As an example, if you have a ton of pets (granted they are not hairless) running around in your home then you will need to change the bag more so than an individual with no pets.
Additionally, if the majority of your vacuuming is on carpeted flooring it can fill up quickly. For this reason, you will see a wide range of vacuum cleaners designed with an indicator for this issue. If you are lucky, it will inform you when it is time to replace the current bag.