Went through Dulles last year transferring from bmi onto United. We arrived at Terminal C, which also looked to be where the international United flights parked (there was a UA 744 next to us). We cleared immigration down in the bowels on C, (took 45 minutes, and I reckon there was a third of the pax off our flight behind us. Unfortunately we then had to wait another 45 minutes for our bags to arrive, as a container of transit bags had been sent to the main terminal, rather than the transit area, and it took that long to get them back. I got the impression that this was not unusual at the time. However we were on a 2 hour connection to Miami (also from C), and we made it with a few minutes to spare. Unlike an experience at Atlanta a couple of years ago the security check after customs had no queue, and so that didn't delay us - and the staff were friendly and courteous too which was a bonus in this day and age.
Best of luck!