During daytime, there's an airside shuttle bus between the B and C piers. It doesn't require an extra security screen. And USAir uses both piers, so you might not have to change.

If you choose to walk between piers, the wait depends on the time of day. Assuming your inbound flight is on time, I wouldn't be too worried about it. Security at DCA is generally uncomplicated.

Might consider a move of this question to the Washington DC forum.