Step 3. Hang your grow light based on how powerful it is. Less powerful High Intensity Discharge (HID) lights should be hung 2-3 feet above the tops of the plants, while more powerful systems should be hung four to six feet above the tops of the plants.
http://www.gardenguides.com/69975-use-grow-light.html
Efficient grow light stands bring the sunshine indoors. Grow lights are perfect for indoor seedstarting. You can also use them to grow light-loving houseplants, such as orchids. Grow lights, such as this Compact 2-Tier SunLite Garden, are attractive enough to have in the kitchen. Our Vermont-made line of SunLite Gardens includes both standard (4 feet wide) and compact (2 feet wide) models.
http://www.gardeners.com/how-to/gardening-under-lights/5080.html
The 240 Watt LED Grow Light should be 16-30 inches from the top of the canopy. Larger lights such as the 450 Watt LED Grow Light should be lowered around 20-30 inches from the top of the canopy. More intense lights such as the 600 Watt LED Grow Light shold be kept at 24-26 inhes & the 900 Watt LED Grow Light should be kept 26-42 inches from the top of the canopy of your plants.
http://www.dormgrow.com/grow-tips-for-led-grow-lights
You can even keep the plants under 24 hours of light and they will continue to grow in this stage. However, the recommended growth cycle is 18 hours of light during the vegetative growth stage. This light cycle will create an enviornment that mimics the photoperiod in the summer with long days.
http://www.dormgrow.com/grow-tips-for-led-grow-lights
This tells you how powerful a light you need; the bigger the area, the greater the wattage you'll need. As a general rule, a 250-watt grow bulb will light a 2-foot-by-2-foot area; 400-600 watt bulbs will light a 4-foot-by-4-foot area, and a 1,000 watt bulb will light an 8-foot-by-8-foot area.
http://www.gardenguides.com/69975-use-grow-light.html
As with the vegetative state, when your plants are seedlings the light should be at the recommended hanging distance. As the plants start to grow adjust the light upwards a few inches at a time. Protect Your LED Grow Lights. Buying a LED grow light is a capital investment for your indoor growing hobby or business.
http://www.dormgrow.com/grow-tips-for-led-grow-lights
There is simply no better way to grow an abundance of stocky, green seedlings. If you grow flowering plants indoors, such as orchids, African violets, citrus and hibiscus, lights will keep them blooming almost year-round. A wintertime harvest of herbs and salad greens can also be grown with lights. By learning how plants use light and about the fixture options, you can select an indoor lighting system that is right for the plants you want to grow.
http://www.gardeners.com/how-to/gardening-under-lights/5080.html
2) Grow plants that are out of season or that you are overwintering indoors and need more sunlight. 3) Grow plants in dark rooms like a garage or basement. I hope this primer on using grow lights for your indoor plants has helped you understand how to select bulbs.
http://foodiegardener.com/grow-lights-for-beginners-start-plants-indoors/
Step 4. Use a basic light timer, available at any home improvement store, to run your grow light on a regular schedule. Most plants will require about 12-16 hours of light per day. They also require at least 8 hours of darkness each day. Start by setting the timer for 12 hours of light and 12 hours of darkness and adjust the light time upwards if your plant does not appear to thrive.
http://www.gardenguides.com/69975-use-grow-light.html
LED grow lights are great for cloning plants. Proper usage of LED grow lights will multiply the number of plants you have in a methodical and environmentally friendly way. Cloning require only 2-3 weeks in the vegetative state. Use an 18-20 hour ON cycle for your clone and spray it with water intermittently.
http://www.dormgrow.com/grow-tips-for-led-grow-lights
how long should a grow light be to seedlings
Seedlings should be grown within 12 to 16 hours of light per day.   A grow light should be for 12 to 16 hours per day for seedlings.