Unless it is an auto flowering strain the plants should stay in their veg. stage until the darkness is increased. Male plants may show their sex with no change in the light cycle, females may show pre flowering. You can run the lights 24/7 for a few weeks to speed up their growth cycle then scale it back to 18-20. I believe a plant will start to flower with only 10-11 hours of darkenss, I've been giving mine about 13 hours and the bud size seems to have increased condsiderably in the last week.