One of the observations from the studies conducted so far is that the effect of light on the prevention of the development of myopia may have a threshold effect, that is both the level of light required and the duration of light exposure may need to reach critical amounts before light has its preventive effect.
In the latter case, the epidemiological studies that have examined children’s exposure to outdoors have consistently found a preventative effect for between 10-14 hours outside per week in addition to any hours spent outside during school time, while 3-6 hours per week has not been associated with any effect.
While I did not log my hours at the time, I do know that I was playing outside for well over the 14 hours per week needed for the theoretical preventative effect, and my eyes still became increasingly near-sighted as I grew. Obviously the study could not account for all factors affecting eyesight, but at least there’s hope for some children to prevent or slow the development of myopia.