How long a rash lasts depends on its cause. However, most rashes usually disappear within a few days. For example, the rash of a roseola viral infection usually lasts 1 to 2 days, whereas the rash of measles disappears within 6 to 7 days. Rashes caused by an antibiotic allergy may last 3 to 14 days, whereas diaper rash almost always clears up within 1 week (if diapers are changed frequently).
Rashes resulting from lupus or dermatomyositis may last for an extended period of time.