You'll get more growth with them on 24/7. How much more I'm not sure. I know that you can grow annual terrestrial plants on a 24/0 cycle then switch to a 12on/12off or a 8on/16 cycle to induce flowering. I guess it depends if your willing to spend the extra money on electricity and fertilizers. I've forgotten to cycle my timer and my lights were on over Thanksgiving break. Talk about GROWTH! Is this good for the plants in the long run? More than likely not.
I'd still do 12hrs a day, more than that and you're asking for alagae problems.
Also, a suggestion, if this is just a grow-out tank, try placing mirrors at the back and sides of the tank. Its not pretty, but instant, cheap up in your WPG.
If you really want to increase the reflection, try some mylar. You can buy it at wal-mart. Look in the camping section. It's sold as a cape thinger for keeping warm. Mirrors will work a little, but if you look under the glass it's black. And black absorbs light. Mylar on the other hand gives you over 90% reflection of usable light.