This will depend on the type of plant you are growing and on how much natural light your plants are getting. As a rough rule of thumb to effectively mimic the sun's natural light it should be on for 8- 16 hours a day however if there is already a lot of natural light, it can be used for just a few hours a day to top up the light and keep the plant thriving.
Even plants need a good nights sleep! Make sure not to run the light for longer than 16 hours a day so your plants can rest.