Benadryl has a half life of 4.3 hours. Typically, I find one pill per 70 pounds of me is enough to knock me out. For a 140 pound person on a 7 hour flight, I'd take 5 as you get on the plane.

(Keep in mind that regulations wrt Benadryl differ country-to-country, and that using too much too often does have negative health effects.)