Root Expansion timing?

so right now i have 12 sprouts all about 3" tall with 1 set of serrated leaves
i have them on 16:8 photoperiod under 2 40w t12's for each 6 sprouts
how long should i expect b4 their roots stretch out and not down and should i transplant b4 they are root bound?

It depends on how far they have to go down. I like to put my plants into their final resting spot shortly after they sprout....Sometimes I just start them in the container I am finishing in.

the sprouts have about 4-5" of soil beneath them but i can see the roots through the bottom of my clear plastic boxes but they only have tiny offshoots of the main root
could the roots not be gettin enough water or am i just paranoid cuz they have only been getting light for about 10 days

If you see tiny roots at the bottom you can wait a little longer. Just dont wait to you see a gigantic root ball lol.

how long does it normally take for a plant to become root bound in a box thats 4"x4"x5"?