I have lights on for longer than 12 hours during the summer time, since that's when the longer days are. During the summer times, I tend to have the lights on for 14 or so hours.
I set my lights pretty close to when the sun sets and comes up and adjust with the change of seasons. I might have the lights come on a little later in the morning and go off a little after dark but try to mimic what it is like outside.