How long

OK my plant is only a week in a half old. These pictures are from a couple days ago but i was wondering when do i start turning my light off for 18/6? how tall? also my bottom leaves are healthy but there about to touch the soil. its not but like 2 inches It has 16 leaves already.How do i make it grow up instead of out?

what do you mean by turning the lights of for 18/6? did you grow those seedlings really on 24/0??? oh my.
put them on 18/6 now, plants need the dark period to grow, roots grow only at night for example and strong roots are essential for a good harvest, they are your plants foundation really so put them to rest, catch some beauty sleep

There is nothing wrong with having seedling under 24 hours of light just switch them now. No biggie. Now the standard questions. Do you ph test all your liquids before adding them to your plants? What is the temp of your grow area? What ferts are you using and at what strengths? Do you have a moisture wand? What kind of light(s) are you using and how close to your plants are they?