How long can a light be on for a tropical fish tank each day?

I would say that it is up to you. As you have no real plants they don't need the light rays to photosynthesize. This means you don't have to worry about how much light the plants need. Remember that the light encourages algae growth. The longer it is on for will determine how quickly and how much algae grows. Bear that in mind but it is up to you. I put mine on in the evenings or as soon as it begins to get dark until i go to bed. DO NOT leave it on all night. Hope this helps.