As Andrew notes, the fit of the respirator is as important as the filter cartridges. There should be adjustments for all straps - properly adjusted, a good respirator is comfortable to wear. To test for fit, first adjust the straps, then cover the cartridges with your hands and inhale. You should feel the body of the respirator pull tight against your face - if it doesn't, there is not a good seal between your face and the rubber. If you have a full beard, you may not be able to get a good seal.
For cartridges, use the two-stage type, often called a combination cartridge. Inside is a paper filter element, which removes the particulate matter (paint dust) and an activated charcoal element which removes the harmful vapours. When the paper becomes clogged, you will have difficulty getting enough air through the cartridge, indicating that it's time for fresh ones. If, while painting with lacquer-based paints, you begin to smell the paint, the charcoal element is past its "best-by" date, and the filters should be replaced. The ones that I use are from the Mine Safety Appliances Company, in Pittsburgh, and are approved for use with organic vapours (that includes paint fumes), acid gases, and particulates.
Wayne