What is the typical length of time for an allergic reaction?

Depends. An acute allergic reaction to a food, drug or insect sting begins in a few minutes & lasts 2 to 48 hours. Pollen, animal dander, dust mite & mold allergies begin in minutes but last as long as exposure continues. For example, ragweed allergy lasts for about a month. Acute contact dermatitis (poison oak/ivy) lasts 1-2 weeks; chronic (nickel) resolves only with removal of the offending agent.
Varies. Typically severe reactions occur between 15-120 minutes after exposure but delayed type hypersensitivity reactions can occur from 24-72 hours after exposure and tend to be less severe.