The effect of the polarization time in the formation of the PMMA space charge relaxation has been studied. The study has been carried out by thermally stimulated depolarization currents of electrets formed by windowing polarization. The results obtained have been fitted to the general kinetic order model, and they are consistent with the existence of a space-charge polarization mechanism with the kinetic order of 1.14. The intensity of the peak maxima results in being a good indicator of the trapped carrier number evolution. For high temperatures and high electrical fields the saturation of this mechanism is achieved faster, which is attributed to a carrier mobility increase with these parameters. Finally, a compensation law has been found for space-charge-relaxation in PMMA.