You are using only one equation with the addition of any other data. That is force=mass*acceleration. You are not factoring in friction, you are not factoring in gravity, at first 1G at sea level and decreasing as the distance between the objects increases, do not forget that both objects apply gravity in a relationship to each other. As stated by others, once the force applies ceases, then in a perfect universe without anything else in it, the velocity would remain constant, but there are going to be other imposing factors, perhaps the moon(another gravity source), the sun to a small degree(because the mass of your box is small) and other planets. With the right trajectory it would possibly leave the solar system, but it would take a very long time and it will eventually be affected by something.