The lamp should begin to glow in a few seconds and will continue to grow in intensity for a few minutes. This is not a cause for alarm and is part of the normal burn in process. The amount of time your light should be on depends in the intensity requirements of the plants and what type of plants you are growing. Plants can generally be grouped into three categories. Day neutral plants are plants not affected by the length of light or dark periods.
Most vegetables are day neutral.
Day neutral plants flower or bear fruit due to the age of the plant or temperature. Day neutral plants respond well with 18 hours of light, assuming your plants get no sun at all. If you are using your grow light for supplemental light, you may get the same results by turning it on 4 hours before dawn and another 4 hours after dusk. Long day plants are plants that generally flower in late spring or summer when the daylight hours are long. Plants in this group will flower when the amount of light is increased from 12 to 18 hours per day.