To determine the time it takes for a stone to increase its speed from 4.8 m/s to 10.7 m/s using the impulse-momentum theorem, one must consider the acceleration due to gravity. The impulse-momentum theorem relates the change in momentum to the impulse applied, which can be expressed as force multiplied by time. Since the force acting on the stone is gravity, represented as mg, the acceleration can be calculated even without knowing the time interval initially. By rearranging the equation, time can be solved for using the known values of initial and final velocities along with the acceleration. Understanding these relationships is crucial for solving the problem effectively.