It depends on what you need the lights to do. Are you growing plants? If so you need to find a balance between your plant growth, restricting algae growth, and how much tank maintenance you are prepared to do. Normally 6 to 8 hours is what you end up at. Specific time doesnt really matter, have the lights on when you are at home enjoying your fish.
This is my lighting program for a moderately planted, low tech set up. Keeps my plants in good condition, algae growth not too bad, whatever the snails dont eat needs cleaning off maybe once every 2 weeks.