The ferroelectric switching time of a copolymer of vinylidene fluoride and trifluoroethylene depends on not only the applied electric field strength but also the history of the previous polarization process. Measurements using a bipolar on-off pulse field have shown that polarization reversal is accelerated as the pulse width is decreased. The rate of acceleration becomes more significant as the applied field strength is reduced. For a given pulse width, the switching time depends on the applied field according to an exponential law in that the activation field decreases with decreasing pulse width whereas the high-field-limit switching time remains constant. These results can be interpreted in terms of the nucleation probability that increases with decreasing on and/or off pulse width.