Originally Posted by wetrat0

It should take about an hour by car to drive from DCA to IAD at 9 pm. Whether 3 hours is enough depends on whether you are checking bags (if you are then I personally wouldn't try it) and crucially, whether your inbound flight is on time.

Do not attempt this transfer by public transit. Metrorail is unreliable lately and service is usually reduced quite a bit after about 7 or 8 pm. aztimm quoted a post stating that a cab would be $50-60. I would expect this to be on the low end for Uber/etc. In a regular cab or if there is traffic expect to be closer to $75-90.