Washington Dulles (airport code: IAD) is about 25 miles outside of Washington DC.  Washington Reagan (formerly known as National), on the other hand, is maybe a mile across the Potomac River, and visible from the Capitol or the White House.  Don't get the two airports confused.  If you need to drive to Dulles (many rental car agencies will let you drop off your car at either airport without a surcharge), take the free Dulles Airport Access Road.  The Dulles Toll Road runs parallel, but you'll need to pay a toll.  If you accidentally get onto the Toll Road, there are several onramps from the left lane of the Toll Road that will give you access to the Airport Road.  Give yourself at least an hour to get there from DC, and another half-hour if you need to return a rental car, as the drop-off location is about a half-mile before the main terminal.

If you're planning to take public transportation, you can take the Metro to L'Enfant Plaza or Rosslyn and take the  5A Metro bus to Dulles for $3.  Note that the 5A line, originally intended to provide inexpensive transportation for airport workers.  It was NOT intended to be used by tourists and business travellers trying to save a buck, who now overcrowd the vehicle with their bodies and numerous pieces of luggage.  If you're not able to board at L'Enfant Plaza or Rosslyn, you've got an hour wait until the next bus.

If that happens, take the Orange Line Metro to West Falls Church and catch the Washington Flyer connector to Dulles for $9.  Or if time is of the essence and you've got money to burn, a you can get a cab or shuttle to take you to Dulles for upwards of $50.  Note that a shuttle may pick up additional passengers at other hotels and pickup locations before heading to the airport, so make sure the driver knows your flight departure time.

Getting to the airport is only half the battle.  Once you arrive at the terminal, be prepared to wait in long lines at the ticket counter at peak times during the week, with little direction or assistance from airport personnel.  If you have an international flight on United (or on any other Star Alliance member), the ticketing booth is actually on the REAR side of the ticket counter.  You could waste an hour in the main ticketing line only to find out that you should have been in the other!  Afternoons and evenings are often the worst (particularly for international departures), as time zones make this the optimal time for wide body aircraft to depart for Europe.  Once you've received your ticket, you may need to wait in another line to drop off your luggage at the screening station, as the ticket agents no longer check your baggage at the counter for most airlines. 

Once you've deposited your bags, you'll be directed to what can be another long line to pass through the TSA security checkpoint.  Unlike Washington Reagan which has a separate checkpoint for each of its three piers in its main terminal, Dulles has only ONE for the entire terminal.  As of the fall of 2009 a new security area has opened, with as-yet to be determined impacts on lines.  With the old setup, although there are several lines, it may move excruciatingly slow, especially on Mondays and Fridays.   Then, after you've had your luggage and your shoes closely examined, you'll need to board a shuttle bus.  If you're lucky, you'll be one of the last standees squeezed onto the bus and it'll depart for the mid-field terminal.  If you're one of the first to board, however, depending on which shuttle you could be cooling your heels for ten to fifteen minutes as the driver tries to fill every square inch of floorspace with passengers.  At peak times the wait for a shuttle is a more reasonable 3 to 5 minutes, depending on your departure terminal.  Then it's a short ride to the midfield terminal, followed by, at most, a half-mile walk down the corridor to your plane.

Bottom line?  Budget enough time to get from DC to the front door of your plane.  Early mornings and at off peak times, leaving downtown Washington at least two and a half hours prior to departure for domestic flights should be sufficient.  During evening rush hour, at holidays, or during bad weather, add an hour to ninety minutes to allow for additional traffic and security delays. Add another 30 to 60 minutes for an international departure leaving at a peak time.  It's better to arrive at the gate early and catch up on your reading, than to arrive "on time" and miss your flight (most carriers there secure the plane doors five minutes before departure time!).  While it is possible to walk through the front door of Dulles 60 minutes prior to your scheduled departure and miss your flight, for domestic flights at all but the worst times of day entering the airport terminal 90 minutes prior to departure is typically sufficient.