Actually, United's website has a page specifically for IAD here. It says the average wait time at security between 6am and 8 am is 45-60 minutes. Combine that with the people mover and the construction and you really need to budget more time.

By the way, their website also says "Please note that reservations are subject to cancellation if you are not checked in and available for boarding at the gate at least 20 minutes prior to departure between U.S. cities." I suspect you already saw that, as it's on the same page that states the arrival 60 minutes prior to departure.