We introduce a model for the kinetics of grating formation during holographic recording in optically bistable systems. An unexpected characteristic time dependence of the diffraction efficiency is found. We show that it originates from a nonlinear transformation of the light interference pattern into a refractive-index profile. Our findings strongly resemble and explain by nature the experimental data of optical bistable systems in general, here exemplarily presented for sodiumnitrosylprusside and terbium gallium garnet.