You're taking a big risk on traffic. If all goes well, you should be in the airport parking lots within an hour or so. But heavy rush-hour traffic (which is the norm) will add 3045 minutes to the ride.

You seem amenable to public transit. The cheapest way is taking Metro to Greenbelt and then the B30 Metrobus. That avoids the Beltway traffic jams, but you are still susceptible to traffic issues on I-95.

Best way is take Metro to Union Station, and then the Marc train to the BWI station. Shuttle bus than takes you from the BWI rail station to the airport terminal. Amtrak trains cover the same route, but costs about 3 times as much as Marc commuter train. Either way, you still need to allow time for the bus-train-bus - but you shouldn't encounter any traffic problems this way.