Change in speed would continue as long as an unbalanced force is applied.
Explanation:
When an unbalanced force is applied to an object, the acceleration of the body changes according to the equation , where,
m is the mass of the body on which the force is applied and
a is the acceleration of the body on which the force is applied because the mass of a body is constant and cannot change.
Therefore, if an unbalanced force is applied, the speed change will last as long as the force is there.