Car air fresheners typically last anywhere from a few weeks to a couple of months, depending on the type and brand. Hanging cardboard air fresheners, like the popular tree-shaped ones, generally last about 2 to 4 weeks. Gel-based air fresheners can last between 4 to 6 weeks, as they slowly release fragrance over time. Vent clip air fresheners, which attach to the car's air vents, usually last around 30 days, but this can vary based on air circulation and usage. 
Canister or tin air fresheners, which contain scented beads or gels, can last from 1 to 3 months, depending on the size and how often the car is used. Oil-based air fresheners, such as those in small bottles with wooden caps or reed diffusers, can last from 1 to 2 months, as the oil evaporates slowly. 
Factors affecting longevity include the car's temperature, humidity, and how often the air freshener is exposed to air circulation. High temperatures and direct sunlight can cause the fragrance to evaporate more quickly, reducing the lifespan. Conversely, cooler temperatures may prolong the scent. 
Ultimately, the longevity of a car air freshener depends on its type, the environment inside the car, and personal preference for scent strength.