as Oldman says as long as the fish (and plants) get 8 hours of darkness so they can rest, then they will be fine and you can have the lights on for the rest of the time (up to 16 hours per day). Most people have the lights on for about 10-12 hours per day and see how the plants go. If the plants grow well and you don't get much algae, then you can increase the photoperiod a bit. If you get lots of algae then decrease the time the lights are on.
The other thing is the intensity of the light. If you get 400watt metal halides and put them on a tank that has only had fluoros on it, then the fish will probably stress a bit. Under this type of situation it would be better to have the fluoros on for a bit and then turn the halide on for an hour each day. Every week you would increase the time the halides would be on. The fluoros can be turned off when the halides are on.
The other thing is if you turn the tank lights on first thing in the morning while the room is dark, then the fish will stress out a bit and possibly dash about the tank or maybe even jump out. It is best to open the curtains or turn the room lights on 30 minutes before turning the tank lights on. And if you have more than one light on the tank then turn one on, wait 30-60 minutes and then turn the other on.
At night turn the room lights on, then turn the tank lights off. Leave the room lights on for 30 minutes before turning them off. This gives the fish a chance to wake up and go to bed without being subject to sudden bright light or darkness.