Change the bag when it is 2/3 full
For bagged vacuum cleaners, the dust bag must be regularly replaced to prevent over filling and loss of suction and to generally encourage maximum levels of performance from your appliance.
Tell-tale signs that the bag is full include a noticeable loss of suction and the visibility of debris or dust on the floor despite cleaning. For some models, there will be a bag check indicator, which will automatically alert you to the need for a new dust bag. Always use genuine Hoover bags, please view this video clip for more information regarding paper dust bags: