Photoperiodic Indoor plants in the vegetative stage should ideally be receiving 18 hours of light and 6 hours of darkness in a 24-hour time period. This is represented as 18/6. This is only a recommended average; with some strains requiring more or less light or darkness.
In case of cannabis outdoor growing all the above conditions happen naturally from mid of spring till the end of summer when daytime is longer is referred to 18/6 light/darkness cycle period (vegetative period of cannabis). The period from the end of the summer till the end of autumn is referred to 12/12 light/darkness cycle which is flowering period of cannabis.
When indoor-grown plants move into the flowering stage, the light period, or their photoperiod needs, change from 18/6 (Vegetative Period to 12 hours of light, 12 hours of darkness to let is start flowering..