No stupid questions... only stupid people... o wait, sorry, didnt mean anything by that.... haha
But this isnt a stupid question and it's not as obvious as many people make it to be. Lighting is as particular to your set-up as you want to make it. For highly planted tanks and marine reefs with photosynthetic corals, long light durations throughout the whole day are good to grow the life big and strong. In some types of tanks, low light only on for a few hours a day, maybe 5 to 7 hours is fine as the fish naturally live in a dark or muddy environment where there isn't much light at all.
Anyways, to actually answer question, you can turn the light on in the morning, maybe 30 minutes to an hour after sunrise to allow ambient light to wake up everybody and get the tank moving before the bright lights turn on. And then, turn the light off around dusk, maybe a little after, maybe before. This should help regulate a daily cycle for the fish. The best thing to do is to buy a timer that will do this automatically for you incase you forget one day and the light is off all day or on all night and the fish become stressed because they think their world is ending.
