My examples for DCA were during periods of an operation at least every 2 minutes.
I'd guess they are averaging about an operation every 90 seconds for hours on end. All their runways intersect and the jets mostly use the long one. The airport has so little ground area that planes don't sit around for long times between flights. They are generally in one hour, out the next.
See
http://www.fly.faa.gov/Products/AADC/aadc.html for an example of arrival traffic demand. Double click on DCA. Time shown is UTC/zulu.