Normally people suggest anywhere from 6 to 12 hours per day, but every tank is different. You can leave your lights on as long as it doesnt produce algae. If you want to increase your lighting times start with increasing it little by little. increase the schedule by something like 30 minutes, wait a week or two, and monitor for signs of algae. when you reach your desired duration then stop increasing the time, or if you begin to see algae growth cut the time back by 30 minutes to an hour. I have my lights on for 9-10 hours per day, but my live plants and close monitoring of fertilizer helps me walk that fine line before algae