How long can the rash of slapped cheek disease last?

Slapped cheek. Slapped cheek is the first stage of 5th disease, which is a common viral infection of childhood and lasts 2-5 days, then a lacelike rash begins on the arms. The rash can recur because of heat or exercise over weeks/months after the initial rash fades. The infection is self limited and no treatment is necessary, but if your child seems sick, always best to see your pediatrician.
Apparently... ...The record is 4 weeks, but it is rare beyond a week.