Based on time-averaging effects for atmospheric scintillation, extending photoelectric detector's exposure time is one of the effective ways to mitigate the effect of atmospheric scintillation on the link of atmospheric optical communication. Based on the Kolmogorov spectrum, the expressions are derived for time-averaging factors associated with atmospheric scintillations of unbounded plane wave and spherical wave, in the weak turbulence regime. Approximate expressions of the time-averaging factors for small- and large-aperture receivers are provided by use of geometrical optics approximation, respectively. When the aperture is much larger than Fresnel scale, approximate expressions differ from exact numerical results by less than 6% in the case of plane wave and 3% for spherical wave. Results show that the strength of time-averaging effects goes up along with the aperture decrease, and the effect of Fresnel scale on time-averaging effects weakens along with the increase of the aperture.