The Airlines recommend that you arrive at Dulles 3 hours before your international flight or 2 hours for domestic. Dulles can be an extremely busy airport especially during holiday periods and outside of the NY airports it is the busiest international carrier in the Mid Atlantic. During such periods and when the weather is especially inclement please allow extra time to get to the airport. It's worth knowing that security screening is available 24/7.
You can get an advantage and get smoother processing to reduce your TSA security wait times at IAD with TSA PreCheck membership. Members benefit from a dedicated lane for identification and verification by TSACheck and simpler processing e.g. can leave shoes on, leave a laptop in your bag etc. Check out today's estimated TSA PreCheck wait times at Dulles Airport above in addition to open lane info. To find out more about joining or renewing TSA PreCheck membership visit and the associated benefits visit https://www.tsa.gov/precheck.
You should not expect long wait times at Dulles International customs screening. The airport does receive more international arrivals than neighboring DCA Airport so if arriving in close proximity of other international arrivals queue times can sometimes be lengthy. For passengers connecting at IAD you will need to collect and re-check your bags before onward travel.