Use your grow light for 12-16 hours at a time. Seeds, seedlings and sprouts need less light, about 12 hours. Bluer shades of light work for these young plants that are concentrating on growing leaves. Flowering plants need closer to 16 hours of sunlight. They prefer red light once they're ready to flower. Full-spectrum light is good for all plant stages because it contains red and blue light. For best results, research your specific plants to find out exactly how much light they'll need.