Set your grow light on a timer so the plants get the same amount of light every day. Consistency matters more than the exact hours, and plants need a dark period too.
Start with the plant
Leafy greens and herbs are often grown with a long daily light period. Check the guidance for the plant you are growing and adjust from there.
Use a timer
A plug-in timer is inexpensive and removes guesswork. Set it once, then check the schedule every few weeks.
Watch for signs
Stretched, pale growth can mean not enough light. Scorched or bleached leaves can mean too much light or heat. Adjust the schedule or the light height.
Keep light off the reservoir
Light reaching the nutrient solution encourages algae. Use an opaque container and cover any exposed water.