Keeping the tank light on for about 12 hours each day is the general rule I've always gone by, and the main reason is to reduce the chances of excess algae growth. Algae benefit from light just like plants, so longer periods of light mean the potential for more growth.
However, if you have live plants in your tank, you'll want to keep the light on for at least 12 hours per day. Obviously, this is because plants need light to grow and be healthy.
They also out-compete algae for food sources, and well-maintained planted tanks tend to have less algae.
Finally, both plants and fish benefit from a day/night cycle, such as twelve hours lights on / twelve lights out. Just be sure to keep a regular schedule.