preflowers will begin showing once alternating branching gets going...or about where you're at now. it begins anywhere from 5-8 weeks and signifies the plant is sexually maturing and you should be able to see them at the joints of branches with trunk (i tend to find them 1st at the junction where the pinch was made)...and later, up on the branches where the leaves meet them. a 10x magnifying glass will help immensely with id-ing them.

and if i were you i'd bump up the daylight hours a bit to maximize growth. the least amount of daylight people i know about use is 18 hours while most of us use 20-21 during veg.