Depends on how big your tank is and how strong your lights are. Generally in a lowlight low tech tank, 8 to 10 hours on, is recommended. There is a watt per gallon rule (guideline) that says 1 to 1.5 watts per gallon is appropriate low light for plants. If you have your light on, your plants will be photosynthesizing and using up nutrients and co2. When off your plants are at rest. More light= harder working plants (growth etc..)
Hope that helps.
Timers are an excellnt way to ensure that your planted tank gets a constant stable light cycle which is best for happy plants and fish
Steward of the Tank