In the case of children, about 30 minutes a day. In the case of adults whose crystalline lenses have been hardened, twice a day 20-30 minutes each will be good (muscular exercise of the ciliary muscle that was hardened by frequent use).
The eyesight training gets rather quick results in the case of eyesight of 0.4 and above, which can improve easily in general. There are a great deal of differences among individuals; given hard work and patience, however, generally 0.5D and above can be improved in one month for myopia including astigmatism, and 0.25D for myopia excluding astigmatism. If 3D are exceeded, the focal spot size spreads geometrically, and it takes a longer period of recovery. For example, for 5D, 5/0.5 = 10 months. But the arithmetical calculation should not be considered alone. Generally, the period can be presumed in case of steady training for one year and half. Some having great concentration overcame more than 1D easily within one month, and took two months, which, however, is a special case. The period should be added or deducted according to concentration.