(Keep in mind I am an amateur planefag)
Not often, whenever I check at night, theres usually a tanker flying around somewhere on the east coat, and usually I see a plane up near the peninsula area of Washington from time to time (the state), and then maybe two or three other aircraft that are patrolling or doing whatever.
6 blackhawks at once, at night, is unusual.. usually you see that stuff in the daytime when there is more daily movement.