I'll make this breif. When you are "growing" the plant its best to have 18-24 hours of light IMO. When you get ready to "flower" you change the light to 12/12, depending on the strain you are growing will depend on the time that you need to keep the plant in each stage of growth.
You can search the site here and you will find pictures of trichomes and when they are ready, but its all personal preference. I like to harvest with 50% milky 50% amber so you get a good head high, and a nice body high as well...answer your question?
There is so much more to growing weed indoors than just providing light. You need the proper spectrum of light, adequate lumens, ventilation, the proper soil & nutrients, all in a chamber/closet.
And growing a crop takes at least 3 months, start to finish.