Marlboro_2002. It depends on your configuration. You can fit up to 9 tanks in the forward and aft holds. All the tanks are of a slightly different capacity, but a quick calculation from my old notes show that they average out at about 3300lbs each, if you have aux. tank fuel then your fuel consumption will be approx. 5500lbs/ph so each tank is worth about 35-40mins(But please don't go off flight planning a round the world trip based on my post!).
EMB170 is quite correct, the BBJ1 can lift the fuel but you have very little hold space, the BBJ2's DOW is usually so high that it can't lift as much fuel but at least you have more hold space. As with most things in aviation it's all a compromise!
HTH