It depends on how much you use the vacuum cleaner and the capacity of the bags. The type of dust that you vacuum is another factor that you should notice.
Cleaning pet hairs and carpets make the bag fill faster.
A small bag should last at least 1 month and a large one spans around 2 months.
If the bags are washable you can wash them and re-use them. If they are not, you have to use a new one.
You may often not get aware of the bag’s fullness. It can facilitate the suction power of the machine.
Nowadays some of the vacuum cleaners are incorporating the Full Bag indication. It lets you know whether your bag is filled or not.
This feature is a good one to maintain the fluency of your machine. Therefore, you can search for a vacuum cleaner that has this feature in it.