I wrote an article about it
here if you're interested. I did a little experimentation with this a few years ago. I found I got the best results by giving them 18-24 hours of light a day for the first 3-4 weeks, and then switching to 12/12 at that point. Even if you give them 12/12 from the beginning, they won't start flowering until around week 3-4 anyway, but they'll grow a lot faster/bigger if you give them more light in those crucial first weeks (without adding to your final time to harvest).
I found it best to wait to switch to 12/12 until plants are about this size or a little bigger (
note: young plants like this can and will
double or triple in height after the switch to 12/12)
At harvest, 3.5 months from seed
Sea of Green in action, with many plants
But if you want the
fastest yields you might consider an
auto-flowering strain. They will usually be quicker to harvest (many auto-flowering strains are ready to harvest by 3 months from seed) and the yields will be higher than 12/12 from seed with a photo, since auto-flowering plants get 18 hours/light a day in the flowering stage instead of only 12. Those 6 extra hours each day gives your plants 50% more light, and your yields will show it! Photoperiod plants tend to get the best yields/results if you let them get a little bigger first
This auto-flowering plant is 3 months from seed. Notice how much more bud is on this plant than the plants above, even though it was ready to harvest in less time
