How long the reaction lasts depends on the type of sun allergy:
PMLE — The rash of PMLE usually disappears within two to three days if you avoid further sun exposure. Over the course of the spring and summer, repeated sun exposure can produce hardening, a natural decrease in the skin's sensitivity to sunlight. In some individuals, hardening develops after only a few days of sun exposure, but in others it takes several weeks.
Actinic prurigo (hereditary PMLE) — In temperate climates, actinic prurigo follows a seasonal pattern that is similar to classic PMLE. However, in tropical climates, symptoms may persist all year round.
Photoallergic eruption — The duration is unpredictable. However, in most cases, skin symptoms disappear after the offending chemical is identified and no longer used.
Solar urticaria — Individual hives typically fade within 30 minutes to two hours. However, they usually come back when skin is exposed to sun again.