The timing of when to turn on grow lights will depend on a few factors.

Duration of Light: 
Most plants require 12-16 hours of light per day. This mimics the natural daylight cycle and allows plants to carry out photosynthesis effectively. However, some plants may require more or less light, so it is important to research the specific needs of your plants.

Time of Day: 
Plants need a period of darkness to rest and recharge. It is best to turn on grow lights in the morning and turn them off in the evening to mimic the natural daylight cycle.

Seasonal Changes:
As the seasons change, the amount of natural light available will also change. It is important to adjust the duration of light provided by grow lights accordingly to ensure your plants receive the right amount of light.