Practically you can have it for as long as you want to.

That's a good question, but I guess it would depend on if you have real plants in it and you'd need a minimum of 12 hours a day most likely to keep plants healthy. As for the fish, I guess if the tank is in a dark location they might appreciate at least some supplemental lighting during the regular daylight hours, but some fish do rest at night so I'd not leave the lights on 24/7. It might depend as well on the fish and some do seek out shady locations during the day and seem to prefer darker locations. You'd most likely have to research specific species to find out what your particular fish prefer. Years ago I had an African knife fish , who would only come out when the lights were out , but I think that was an effort to avoid more agressive fish , who for some reason chose to eat him from the tail backwards and finally killed him. He'd burrow in the gravel during the day and only come out at night though the meaner fish werent' deterred by that habit from eating him.
Not a pretty sight!!

I have mine on timers... about 12 hours a day.

Rule of thumb is 8-14 hours for a live planted aquarium, darkness is important to plants also, photosynthesis. Artificial or non-planted 4-8 hours. Light is an enabler for algae, especially in an non-live plantted aquarium. Also I think the fish might enjoy a little darkness. I keep mine on timers also, just ordinary lamp timers from a hardware store. I try to program the timing when I am home and can enjoy the aquarium the most.