Once a plant has left the seedling stage or the cloning stage, if coming from a clone instead of a seed, it will remain in the vegetative state until you are ready to flower the plant. During this stage you need to provide your plant with a longer time period of light from your LED grow light but the light can be less intense than during flower. Due to this lower requirement of power you can use less watts per square foot and save money on both the cost of the LED grow light as well as the savings from the more efficient LEDs.
During the vegetative period we recommend 16 or 18 hours of light per day. How long you remain in vegetative state before going into flower will depend on many things such as how large you want them to be at the end of flower, the particular genetics you are working with, and your style of growing (sea of green, screen of green, etc.). Other external forces can affect the duration of veg can be items such as a relatively short ceiling which can require you to keep plants smaller.
For most gardeners 4-6 weeks is a good rule of thumb for the veg stage with their plants. This time allows the roots to develop and fill out the pot. This is also when you would do your repotting into larger pots. A larger root mass in a larger pot will increase your yields. When repotting it is best not to jump from a small pot to a large pot. You should take intermediary steps so the roots can develop in the new pot before moving to a larger pot. This will help avoid issues with overwatering. Also note, it is never a good idea to transplant to a new pot right before going into flower as the transplant will shock the plant and you always want to go into flower with a strong plant that has recovered from any transplant.