I turn them on around noon and they're on till around midnight. In the morning there is light from the windows. Also like Egon said before in summer the lights go off during the day too when it's too hot to try and keep the temp down.
8am-7pm on planted tank... everyone else is in a basement that gets indirect sun-light... and my nocturnal knife is located furthest from the light. I have simple strip lights on the tanks if I "Need" to see them better.. and lighting in the basement itself that goes on 'n off as people need to use it.