It looks like you want to do a drop test. While a physical drop test is done by releasing the object with zero velocity at time 0 from a height h, that is not how a drop test is performed in simulation.
To simulate a drop test, you go into CAD and you move the cube so the point is just touching the plate. Then in Explicit Dynamics (or Transient Dynamics), you assign the Initial Velocity, v, to the cube at time 0. Then the solver integrates the dynamic equations to show you what happens from the instant of impact onward.
Do you know how to calculate the Initial Velocity, v, from the drop height h? You may have learned this in high school Newtonian physics class. v = sqrt(2*g*h) where g is 9.8 m/s^2 and h is in meters and v is in m/s.
If you just want to know the time, t, in seconds it will take for the cube to hit the floor, t = sqrt(2*h/g).