Good rule of thumb w/ nutes - don't add until the plant has at least 3-4 sets of true leaves - and that's if your medium contains no nutrients. If you're growing in hot soil like Fox Farm Ocean Forest you don't need to feed a plant until they're almost ready to flower. When do you start feeding remember to start small and conservative. It's 100x better to have a plant thats slightly underfed than one that's overfed/nutrient burned. Overfeeding is pry the biggest and most common mistake amongst new growers.