Light Position?

I have a 18inch fluorescent light with a 16inch light bulb tube in it I have it hanging over the plant but how tall should I wait for my plant to get before I put it verticle next to the plant.  I am growing 60day wonder on 24 hour light but it doesn't get very tall.

 

soon I will have a 300w led light to replace the fluorescent light I use for seedlings.