To precisely control the first-order diffraction efficiency of holographic gratings recorded in an amphiphilic diblock liquid-crystalline copolymer P40A37, effect of recording time on grating formation and enhancement was studied systematically. Several parameters such as surface relief, the first-order diffraction efficiency, the maximum transmittance under an optical polarizing microscope and order parameters of grating samples were evaluated before and after annealing. Then the enhancement coefficient with recording time was estimated. The best enhancement effect was obtained at 10-s recording upon nano-scaled phase separation. By precisely adjusting the recording time, the diffraction efficiency of holographic gratings in P40A37 can be finely controlled from 0.13% to 10%.