Light for seedling

You have several options. I personally beleive that you can run them under 24 hour light for the first 4 weeks if you have the ability to run your lights at 25%. After that, you can switch to 18 hours on and 6 off and increase the light to 50%. At the start of Flower you can go with 12/12 at 75% for the first 2 weeks and then switch to 100% to finish. It is important to make sure that your light is at the proper distance based on the light you are using. Most of them provide a guide for suggested distance from the plant based on phase your in. You can also just start running 18/6 until flipping to flower assuming you are running Photoperiod plants.

18/6 is standard, some people do 20/4 or even 24/0 but there is a lot of different schools of thought here. Most people agree that plants benefit from a dark cycle or that the plant can only use so much light in a given day, so 18/6 is the most common. Then there are those who don't think plants need dark period and that 24/0 maximizes potential. Some people are kinda on the fence and go 20/4. Then there are those that mess around with different cycles. I've heard of people going 24/6 or 20/8 just to see. They also do strange flowering cycles or try to mimic the the sun and seasons by slowly decreasing the amount of light every day.

18 hours on 6 hours off