We investigated the temporal dependence of the second harmonic generation from a 50 mu m thick homeotropically aligned cell of ferroelectric liquid crystal SCE9 (BDH Ltd.) exposed to a square waveform field varying between non zero and zero value. The field switches the molecular configuration between the efficient phase matching geometry and the inefficient helically modulated structure. The power of the second harmonic beam responds to the field on a millisecond time scale. The response time of the signal after switching on the field decreases inversely proportionally to the amplitude of the field, while the response time of the signal after switching off the field is almost field independent and has a value characteristic for the spontaneous relaxation of the helical SmC* phase (Goldstone mode). The angular dependence of the modulated second harmonic intensity is almost the same as in the static external field. This shows that a completely unwound homogeneous molecular structure is formed in a few ms after application of the field.