Typically you are trying to gain root structure before transplanting in the field. To my knowledge the earliest from seed this may occur is three weeks. You may want to stretch it out depending on the time and resources you have. Clones typically take about 2 weeks to show good root development. The stretching may occur due to the placement of your lights or the spectrum of light your plants are being exposed to. Here is a breakdown of what wavelengths of light are important at the early stages of development. blue (400-500nm) = responsible for stem and leaf growth (too much = short, too little = tall) green (500-600nm) = vegetative growth/flowering red (660nm) = seed emergence, most important for photosynthesis infrared (660-730nm) = balance with red light to regulate stem growth and flowering response in addition to sleep regulation (longer the wavelength the less energy it possesses) Based on this you may need to increase your blue light or decrease your red light to balance them out.