Well, of course it depends on how much dirt you suck in with it. For me, the bags last around 3 months. 1 person with cat and house.

My Miele Complete C3 hoover has a mechanical fill indicator for the dust bag. This is a rough guide for changing the dust bag.
The rest is done by common sense.

it just depends on whether you vacuum every day or once a week, as a single person I vacuum once a week, I change these bags every 8-12 months as far as I remember but it may have taken longer until it is full ^.^ I would recommend you to buy a box of 4 packs then you can estimate it yourself these things are not too expensive