How long does it typically take for a cracked cartilage in the knee to heal?

It usually doesn't. Unfortunately, injuries to the articular cartilage on the end of the bones usually doesn't heal. A meniscus tear, another form of cartilage in the knee may heal, depending on the type of tear, and location of the tear.
Short term. Conservatively speaking; and without knowing any other information about you...4-8 weeks.