IAD is my home airport - although I've waited a max of 45 minutes during peak hours, in the late evening, the lines are sparse 5-10 minutes. Never flown during Thanksgiving but the lines look worst than they really are. Worst time would be around 4-5pm any given day because of the large transatlantic volume that go out during that time, so if you have a 10:35pm the lines will be light.