In the case of the particulate respirator, you throw it away when it becomes clogged up. You pitch the half-mask respirator for gases and vapors when its special filtering element is exhausted. Usually, you'll know that has happened when you begin to smell or taste the chemical. The so-called nuisance or comfort masks sold at the hardware store, by the way, really don't qualify as respirators. The two types of cartridges are often used in tandem.
Some cartridges do double duty.
OSHA requires manufacturers to color-code cartridges according to the substances they rebuff. A black cartridge with a gray pre-filter, for instance, protects against particulates and organic vapors. As a result, contaminates may leak inside the respirator. A variation on the reusable half-mask is the full mask, which protects your eyes and more of your face from splashes and flying particles. Full masks also fit more snugly, minimizing the chance of leakage.