How long does an allergic reaction take to kick in?

Usually fast: Most common allergic reactions to pollen, insect stings, foods, etc., are immediate hypersensitivity reactions, meaning they occur within seconds to minutes. But you can also have a late phase, a second-wave of reaction that occurs hours later. Contact allergies (poison ivy, nickel allergy, e.g.) and some medication allergies are delayed, and may take hours, days, or weeks.