*sigh*
to answer your question correctly?..
As soon as your seed(S) crack.. AND the seeds have a tap-root showing.. then you should plant them. U can plant seeds, once they've cracked.. but the general rule of thumb.. is to wait until the seed thats germinating.. rule of thumb is to wait until the seed has a tap-root. Once you see the tap-root.. then plant. (thats what I do personally anyhow).
Also, before I give my plants any light.. I keep them in the dark until they have sprouted or (emerged) from the soil. In other words, until I see the plant coming out of the soil.. they get no light. But once they pop up out of the soil.. and have actually began their life as a plant.. that's when I start giving them light. But until that moment, they stay in the dark.
Just my personal method. There are other acceptable ways to do it..
but generally that's what you should do.
But like some people on here mentioned, they put their plants under the lights as soon as they plant them.. which really isn't the best thing to do.
You really should wait, to give them light.. until you actually see a sprouted plant emerging from the soil to begin its life. And even then, you should
not put them under high amounts of light.. when you first put them under light.. you should start young seedlings (sprouts) under low amounts of
light.. for about the first week to let the plant get used to having light... then gradually work them up to the higher wattage lighting such as MH and
or HPS lighting.
Hope that helped clear some things up..
peace.