First off, as a new member, welcome to Tropical Fish Keeping forum.:cheers:
To your question--and some of this might be "old news," but I've no idea as to your level of understanding, and the complete picture is essential to understanding it.
With any light, there has to be a balance with the nutrients. Plants grow by photosynthesizing, and they can only photosynthesize if the light is of sufficient intensity to drive photosynthesis, and all 17 nutrients they require are available. As soon as something in this equation is insufficient, photosynthesis slows and may stop altogether, and algae is quick to take advantage. So we work to find the balance, which can be different in every tank.
The first questions are, what is the tank size, and what is the length of the light fixture? Jeff, another member, has had good results with this light, but as with any light there are limits.
Next, what plant species?
Last, what fertilizers are you adding? And what is the GH of your tap water [you can get this from the municipal water supply people, probably on their website]?
When we have the above data, it will be fairly easy to suggest a duration that should work for you.
Byron.