Well, that kind of depends on a couple of things:
What wattages are we talking about?
What photoperiods are we talking about?
What plants?
Generally, no matter what wattage you have, 10 hours is about as long as you want to go on your photoperiod. 5-6 hours is probably about as short as you want to go as well. In that range, it's up to you depending on what you want. Longer photoperiods of high light will increase the risk of algae issues. Shorter periods of low light just won't yield a lot of growth.
I had a 45g tank, 192w of light, and a 6 hour photoperiod:
Very intense colors, slower growth, no algae issues.