The appropriate distance between the LED light and the top of your plant will depend on the amount of lumens or light output generated by your LED light. The spacing also involves the type of plants and number of them placed in a given area. Moving your grow-lights can be done either manually or with the assistance of a motorized light moving system.

Manual Grow-Light Movement

For manual movement of your LED grow-lights you will most likely be using a light system hung from the ceiling with a chain that can be adjusted. You can move your lights higher to make the light cover a larger area if your plant(s) have grown large and bushy. You should also lower the light for plants that require the most light exposure to grow properly. You must ensure that the light is as close to the plant as possible without damaging or burning the plant. Fortunately, LED grow-lights aren't considered "hot" and many plants can grow right up to the light without harm. As a rule of thumb, many indoor growers keep their lights at least two feet from the top of their plants and move them up in sequence as the plants grow. Adjust the chain by loosening the links and replacing them to your desired height as the plants grow taller or bushier.

Motorized Grow-Light Movement

To move grow lights on a motorized lighting system you will have had to install a motorized rail apparatus and motor. Typically this system includes a light placed on a rail carrier that can be raised up or down at the flip of a switch on the motor. The light mover is normally raised when the plants grow taller or bushier simply by hitting the "up" button on the motor. The motor may also be set--depending on the model--to move up or down over a given area normally at a pace of 2 feet per minute while pausing for one minute at the end of every cycle. To increase the light area in your grow room move the lights up by hitting the respective switch on your model of light mover. Correspondingly, move the light down when needing to optimize light exposure over specific plants.

Source:

Elights: Plant Grow-Lighting FAQ's

Garden and Greenhouse: Selecting the Best Grow Lights

Little Greenhouse: Indoor Plant Grow-Light Guide