Complex sentence: Unlike contact urticaria in which a rash appears within minutes of exposure and fades away within minutes to hours, contact dermatitis takes days to fade away and then only if the skin no longer comes in contact with the allergen or irritant.
Split into simpler sentences: Unlike contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours, contact dermatitis takes days to fade away. Even then, contact dermatitis fades only if the skin no longer comes in contact with the allergen or irritant.