San Francisco Taxi

How long does it take to travel from the airport terminal transby San Francisco?
I arrived in San Francisco and have Greyhound bus to go to San Francisco International Airport. I can travel on Saturday. Does anyone know how or taxi would be the best transit route would be? Rental cars are not an option. Thanks!
PS transit system mentioned in the first post is called BART (Bay Area Rapid Transit) and know that you are on top of a station where see signs that say "BA" on Market Street. You must purchase a ticket magnetic stripe of the machine used to enter and exit the system. Arriving to the platform you want a train that says San Francisco Airport and Millbrae. The other trains do not go there. Trains arrive every 20 minutes on Saturday. Hope this helps everyone.
Airport Taxi Cab SFO SJC OAK,SAN FRANCISCO BAY AREA