I only leave mine on for a few hours a day. like 3 or 4.
Any more that that and I get too much algae. I usually only have them on when I'm in the same room as the tank too.
Keep in mind that the lights are for your benefit, not the fishes. So it's really just up to you.
Some people like algae so have the lights on 8 plus hours a day.
It depends on the type of light bulb too and if sunlight is coming into the tank or not.