Depends when you are there but there were apparently no waits longer than 15mins over Thanksgiving. In general, I have found few problems and not needed more than 10mins to get through security at most. Of course, if you get caught in a rush, things could be different. However, DCA does not really operate as a hub and have the same scrum that some airports like CLT or ATL have when everyone arrives at once.
Best,
David
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David Lublin