The time response of Bragg gratings is of great concern in the design of fast photonic devices. In this work the time response of gratings written in microstructured polymer optical fiber (mPOF) is assessed using flexural acoustic waves, whereby the strain induced by the waves causes the Bragg spectrum to change. The excitation is achieved by means of an acousto-optic modulator, which is composed by a piezoelectric transducer, a silica horn and the mPOF containing a 2 mm Bragg grating. The time response is assessed using a fast photodetector coupled to an oscilloscope from which important parameters such as the response delay and the grating switching time can be retrieved. The acoustic effect takes approximately 350 μs to change the grating spectrum, which corresponds to the time the standing acoustic wave takes to be generated. The delay observed between the electrical and the onset of the optical signal is related to the inertia of the system and to the time the acoustic wave takes to propagate till the position, where the grating is located. After the excitation reaches the maximum value, the measured stabilization time is around 250 µs. Given such results, a comparison of the time response between gratings written in single-mode silica and in the polymer fiber is performed and discussed, when the same acoustic regime (flexural waves) is applied to both fibers. Therefore, it can be substantially improved by using the long Bragg gratings or by operating at higher frequencies (longitudinal regime).