What assumptions are you making? Are you ignoring air resistance? (If not, then you will have to state what the air resistance is.)
Assuming no air resistance, 2000 m is low enough that we can ignore the change in gravitational force due to change in distance so: the acceleration is 9.8 m/s^{2}. The downward speed after time t is 9.8 t m/s and the distance fallen is 4.9 t^{2}. The box will hit the earth when 4.9 t^{2}= 2000.
The information about the horizontal speed of the airplane (and therefore the horizontal speed of the box) is irrelevant. The time to hit depends only on vertical acceleration and speed.