How often one should replace vacuum cleaner bags?

It depends on the model of the vacuum cleaner. If the model has an indicator that shows the filling level of the bag then one should replace it according to the indicator. If the model has no indicator there are other tests to check if the bag is already full. Usually the vacuum cleaner will get weaker and hoover accurately enough anymore. That will be the sign to change the bag. If the bag is transparent one will be able to check the filling level by taking a look at the inside of the cleaner.