Well according to Alanmin it takes the plants 2-4 hours to realise that the lights are even turned on, from memory he uses very high lighting for up to 18 hours a day on some of his tanks. So just whatever you can afford and feel comfortable with, the longer the better in other words. But 12 hours should be about right. If you can keep the water parameters in balance and use the correct amount of Flourish Excel with the correct redosing periods, thus keeping ontop of algae, you could use even more light hours.
Note: Flourish Excel isn't a magic bullet for "algae", there are many types all with different weaknesses and required parameters, but in my experience if you keep ontop of water changes and Flourish dosing, and have a good plant cover, the algae will slowly deminish over time.
It's a balance between 1) Carbon [eg], 2) Lighting, and 3) Nutrients [eg]. If you combine these three with the correct type of plant then you should be able to grow them easily. As long as these three are in blanace you'll be in control of the algae, if one's out, then some types of algae will grow rapidly.