When to harvest this autoflower ?

For a really clear and mentally expansive high, you could harvest nowish, but if you prefer a sleepy and heavy body stone, harvesting in about 7-10 days would be the time span for that type of effect to develop. Much longer than 7-10 days and the THC levels would have dropped a lot and the THC would have been broken down into CBD and CBN, which are the 2 cannabinoids that give you the heavy and sleepy stoned effect. So it is up to you and which effect you prefer. Awesome looking plant by the way and it really shows how much better a 600 watt HID light is, as compared to a cheap blurple light. Happy toking! ......... Organoman.