How long you leave the grow lights on depends on several plants, but the most important is the kind of plant you have. The general rule is to ensure your plants get 8 – 10 hours of light.
However, there are plants that require plenty of light like aloe vera, spinach, orchids, and lettuce, among others. These will need light for up to 16 hours a day. On the other hand, some plants require plenty of dark time, like tomatoes, pepper, snake plant, kalanchoe, maidenhair fern, and more.
These plants require 4 -6 hours of light. There are also medium light plants that will require 8 – 12 hours of light. Such plants include the African Violet and the Chinese money plant.