This varies tremendously at DL. The nighttime entertainment offerings factor in tremendously, as does the weather! On the most crowded days, it can stay crowded quite late because people are trying to get their money's worth after not being able to do much all day.
When something like fireworks or a parade is happening, you'll see a big dip after that. Otherwise, it just depends. Certainly somewhere between 8-10, you'll see a lot of people exiting the park.
At DCA, they usually close earlier, so you don't see that much of a drop-off until WOC starts, which is sometimes after the park officially closes!