I also echo wldtrvlr's question.

From three month's worth of experience flying out of DCA back in 2003, it never took me more than 10 minutes to get through security at the NW terminal. This was on Friday afternoons, around 4 pm. I could certainly have been lucky, so I would not count on this always being the case for you.

But if you're trying to check in, go through security and get to your gate a half hour before your flight...which means the flight will start boarding as you arrive and you hope to get there before they close the door...all I can say is good luck and I hope you find some overhead space still available for your carryon!