After germination, the seedling has about 3 weeks of nutrients stored inside the seed to provide the plant with what it needs for that time. I would start feeding around day 21 (week 3) very lightly as it is an auto. Autoflowers need alot less nutrients then photoperiods, so start off with a quarter to half of the recommended dose of nutrients around week 3. Any earlier and you run the risk of nute burn and nitrogen toxicity.