Ive been running the light on my 29g from 8am-11am, and then from 3pm-8pm, with seemingly no adverse affects to my fish or plants.
If you are worried about the addition of ambient light during the down time of your photoperiod, and the possible increase in algae as a result, I have found that wrapping the front of the tank with a black garbage bag works.
To do this, I bought some adhesive velcro strips. I adhered two strips, one on the front right side of the tank rim and one on the front left (not on the glass, but on the plastic strip). I then fold a black garbage bag in half and adhere the counter pieces of velco to the bag.
I use this "shade" when I leave the house in the morning to block any ambient light, leaving only my tank light to penetrate. When I get home I remove the shade.
This is also helpful when I need to do a 3 day blackout to combat algae or cynobacteria. During blackouts I actually cover the top of the tank with a bag as well.