How much time does a allergic reaction to hair dye take to occur?

24-96 hours: Contact reactions to hair dye are similar to that of poison ivy although the distribution is usually limited to the scalp, neck, shoulders and sometimes upper chest. It falls under the category of a type IV hypersensitivity reaction. This is different than a reaction to pollen or pets which cause immediate symptoms ( minutes to a few hours) called a type I reaction. It appears in 24-96 hours.