Hey Growmie,
Honestly, it depends on your grow method, the strain, and the phase that the plant is in. On average most growers will start to add nutrients to the soil around Week 3, and your planning on growing them outside start the seedlings indoors after about a or two they'll be ready for the great outdoors. Hopefully, I answered your question and remember ..."Happy Growing" and "Free your mind one puff at a time".