Plants want a definite daily light and dark cycle each day; 10-14 hours is fine; twelve hours is the duration on the equator, where many tropical plants are found.
You should buy a timer ($5-$10) to automatically turn the lights on and off for you, since the plants (and fish) prefer a regular cycle to an erratic one. If the plants need more light, you should not extend the light period, as that will only help the algae. Rather, install another fixture and increase the intensity of light.
