Residual vapors and pigments can also attach themselves to your clothing, hair, skin, etc., and would readily account for the odor you are experiencing. Vapors trapped by the cartridges should be immediately adsorbed onto the activated charcoal. If the vapors are in concentration enough to be given off by the respirator cartridges themselves, I would suspect the cartridges to be supersaturated and should therefore also "bleed" through on the inhalation side of the respirator, signifying the necessity to change the cartridges.
If you have a paper filter, as Berny has indicated, changing it should be predicated on your ability to move air through it. If you have difficulty breathing, it's usually time to change the prefilters. For airbrushing tasks, where minimal amounts of paint are used, and particulate sizes small, prefilters should last for quite some time.
Make sure to keep you respirator and cartridges in a sealed plastic bag when not in use. Moisture can prematurely end the service life of most filter media.
Gip Winecoff