If your LED lights are on too long then algae may grow in your aquarium making it harder to clean and could also make your fish and plants sick. This is why having a timer for your light fixture is a good idea because then you’ll know that your aquarium is getting the same amount of light every day consistently. It may depend on the size of your tank as well as the type of plants you have, but a general rule is to not have the lights on for more than eight hours a day.
However, this is why there are typically different modes that come with the lights. For example, there are usually day and night modes that mimic the sun and moon. This helps your fish get into a better routine and won’t mess up their eating or sleeping habits. This is another reason a timer comes in handy so that your plants and fish don’t get too much sunlight during the night or vice versa. This will boost the photosynthesis process for your live aquatic plants.