Been doing the IAD-LAS r/t about 2 or 3 times a month since January and I've NEVER waited more than 10 minutes to make it through the C gates. The one time I left from the B gates, the wait was even shorter. Maybe I had good luck, or maybe I just try to avoid Sunday evenings and Monday mornings. Remember if C is packed, but D is short, use the D gates and hang a right immediate after grabbing your stuff from the belt. You can walk over to the C gates.