The correct response is: When the force you are exerting on the box is equal to 0.
Equilibrium is achieved when the net force acting on an object is zero. When you stop applying a force (bring it to zero), the box will stop accelerating and reach a state of equilibrium, assuming there are no other unbalanced forces acting on it. If you continuously decrease your force until it reaches zero and no other forces are acting on the box, it will be in equilibrium.