Are you asking when the Plant starts producing the THC?
Mind-bending psychoactive THC is located on the leaves, flower buds, and stalked glandular trichomes. The buds put on most of its weight during the last few days of flowering before harvest. This is why it is critical not to harvest early. Some visual signs that the plant is ready for harvest is The hairs on the bud will turn an amber color and start to fold in on themselves. If you have a 10x - 30x microscope or magnifying glass take a look at the THC trichomes. When they are clear its too early to harvest, when there is a majority of cloudy or amber stalked trichomes that is the peak time for harvest. I hope this has helped you.
Peace and positive energy to you.