There are 3 main ways to make this trip:

1) Catch an Amtrak from BWI Rail Station to DC's Union Station, transfer to Metro (subway) to DCA Airport station

2) Take a bus from the BWI Airport terminal to the Metro. The bus spends alot of time on the expressway between Baltimore and Washington, which can come to a grinding halt for a long, long time if there is congestion or an accident - when you decide to take the trip is key on timing. Forget it during rush hour.

3) Drive. You're at the whim of traffic, which can often be brutal up in that area. Your choice is taking 95 all the way around the beltway to 395 north, or taking 95 to 495 to the GW Pkwy south, or taking 95 to 495 to one of the main roadways and driving it through DC to 395 south.

Option 1) would be at least one hour total depending on your Metro connections and time of day, but the travel time would be consistent.

Option 2) would be the cheapest, but bad traffic while on the bus could sink you - and you have no option to re-route off a jammed road. The bus will 'stay the course'

Option 3) would be at least one hour assuming zero traffic and carries the highest risk depending on time of day...unless you're doing this at night well after rush hour, it's a huge risk