How many hours of light do seedlings need?

Plants do not need light until they sprout their first set of leaves. Having the light on before the plants sprout their first set of leaves will not hurt the plants, it just does not benefit the plants. For most plants once a seedling sprouts it’s first set of leaves it will need 18 hours with the light on & 6 hours with the light off for the grow stage. Some plants have a reverse light cycle (plants that grow in the winter time have a reverse light cycle, they grow with 12 hours lights on & 12 hours lights off and bloom with 18 hours lights on & 6 hours lights off).