Think about two things: Nature and Algae.
In nature, fish see day light for about 8 to 14 hours per day (depending upon where they live and what time of year it is). So if you want to simulate the nature environment, you likely want to have the lights on for this amount of time at a minimum. Once read about an electronic control aquarium a guy put together that even went so far as to electronically bring the lights up slowly over time to mimic sunrise and sunset. After all, going from near total darkness to full light is a bit of a shock to the system for any of us.
But on the other side, if the tank is getting too much light, you are more likely to be growing algae. But algae growth is dependent upon more than just how long the lights are on. The amount of light also makes a big difference. So if you start to have algae problems and you already have the lights on for less than 8 hours per day, then you might need to look into reducing the intensity of light. If you have a hood with screw-in light bulbs, use bulbs with a lower wattage. If you are using florescent lights and you don't need to light for growing plants, try a florescent light with a cooler color temperature (such as those labeled as 'soft white').