Depends on your lights. PC's and MH/PC|T5 combos are usually on a 12 hour schedule for actinics and 10 hours for MH to create the equitorial equivalent of dawn to dusk. My new LEDS do the dawn/dusk by chaning the power to the LEDs.
If you have sps corals you want to follow that 12 hr (aprox) type schedule. Other wise you can go shorter depending on what's in your tank. For a FOWLR you can run the lights just in the everning when your home so you can view the tank.