How long do ringworms generally last?

Many months. Ringworm if untreated may last for months. It is an infection on the skin. Mild cases can be treated with anti-fungal creams. More severe cases will require a prescription.
Not actually a worm. Ringworm is a common name for 'tinea corporis' this is a skin infection caused by a fungus. The infection forms slightly raised, scaly ring shaped spots on the skin. This appearance resembled a worm to someone in the past. . . Hence the name. There is actually no worm involved. Untreated, fungal infections of the skin can last indefinitely. With treatment, they usually resolve within two weeks.