What is the average time needed for an antihistamine to begin to work?

45-90 minutes: Antihistamines taken by mouth begin to work in about an hour but may take longer if you've just eaten. Antihistamine nasal sprays (astepro, patanase) begin to work in 30 minutes. An antihistamine injection, usually diphenhydramine, sometimes given for a severe allergic reaction or anaphylaxis begins to work in a few minutes.

Depends: It depends on the antihistamine, but most begin working within 3-4 hours of administration.