If you do use the economy lot at IAD, give yourself plenty of time to drive over there (if you see a sign saying 'Welcome to West Virginia, you have gone too far, but only by a little ), wait for the bus, and travel back to the terminal. Although the buses run pretty regularly, you can get stuck stopping at several stops before heading to the terminal.

I would give yourself an extra 30 minutes, just to get to the airport (then time for ticket, security, the 'people mover' to the gate, etc.) It can be a lonnnnng affair before you even get to the gate.