I have all my tanks on a timer and they're all set to come on at 8:30am and go off at 8:30pm. I chose that schedule because we can see the fish in the morning without the lights and in general, I put my daughter to be at 8:30pm so there's no point in keeping the lights on if I'm not going to be looking at the tank.
Some people recommend only 8 hours for non-planted tanks. Some break that 8 hours up into smaller blocks of time, like four hours on in the morning, off for four hours and then on again for four hours. I keep mine on 12 hours because 2 out of 4 tanks are planted, and it's easier to keep them all on the same schedule.
-Lisa