Knee Cartilage will not heal on it's own so without surgery the Knee Cartilage will never heal.
Although after you have the surgery for the knee cartilage it should start to heal within 6 weeks once the Knee Joint re-establishes the normal joint fluid.
Although the time it can take to fully heal from the cartilage surgery can vary but most people heal up within 6 weeks of knee cartilage surgery.