I damaged cartilage in knee. What are the best ways to help it heal and what is the average healing time?

Depends. Cartilage can either be the surface of the bone which doesn't heal wlell or the meniscus that heals well but only at its periphery. Some meniscus tears need surgery if they cause locking or catching in the knee.
PT and Time. Meniscal injuries (knee cartilage) injuries variably heal depending on where they are. The more peripheral ones (closer to the blood supply) may heal well, while more interior ones (further from the blood supply) may not heal at all. Rest, avoidance of aggravating activity and pt may help with healing. An MRI is the best way to determine where your injury is. If it is going to heal- 12-20 wks.