I live 5 miles from Dulles and we use the airport fairly often (DH more than me). As long as you don't have to change terminals it's more than enough time, but even if you do, you should be fine. Also, if it's a United Connection, they will be aware if a flight is late and can hold planes for people. It's a much easier airport to change terminals in than say, JFK where you have to ride a bus around making multiple stops. Even if you have to go between terminals, transportation is very good. It's not entirely mobile lounges anymore either, there is a rail system, called aero train that goes between terminals.