Welcome to ILGM. Lots of good info available here. So let’s go down the list…
When the seeds are under ground light is not getting to them, it just heats the soil. Keep the lights at a distance that does not over heat the soil.
In seedling stage you want a lower intensity light, like spring time light. Keep it close enough that the seedlings don’t stretch to it. They should get their second set of leaves before they are too tall to support them if light is right.
During veg you want to up the intensity and move the lights back far enough that you can hold your hand above them with the back of it to the light without burning your hand. If the leaves curl up or get shiny yellow at the center from light burn you’re too close.
Vegging time is anywhere from 30 days on. Up to the grower. Generally, if they are in soil and basic nutes and training, about 45 to 60 days will get you ready. I base it on having at least 16 branches with three nodes each at the canopy level for my Scrog.
There is new info out about light timing. I am currently vegging only 16/8 instead of 18/6. Light to dark schedule. @latewood is down to 14 I think. I am already seeing some relief in my utility bills from the 2 hour drop.
Switching to 12/12 you also want to change the light to fall light, some leds have a flowering switch. After that it will stretch for about two weeks. Once the buds are little balls of white spikes on top of each branch start the switch to bloom food.
Harvesting is another story. Will wait on that one.