to me, 6 hours is a good baseline time. you can always adjust up or down in small increments over time to meet the needs of the plants/tank. if a person ran very low light intensity you can run 10-12 hours and not run into algae issues, but many plants want more/brighter light, and a brighter light is more pleasing while watching the tank. its all about finding the right balance of what works best in each tank. based on what you like to see, plant needs, algae growth etc.