Rashes last for varying lengths of time depending on the type and cause. Certain viral infections can cause rashes, like roseola, which normally last one or two days. Other viral infections, such as measles, have rashes that last six to seven days. Rashes caused by allergies can last mere hours, or in severe reactions, such as a reaction to antibiotics, rashes can last from days to weeks. On the most extreme end, certain conditions like lupus and dermatomyositis can produce rashes that last for years, or even decades.