18/6 to 12/12 ? When?

It's an auto, so you need to keep it on 18/6 until harvest. Some people do 20/4. If you switch to 12/12, the risk is having a small plant and a small yield, plus it will take longer. The best is to stay at 18/6, temperature at 25°C, lights on, 40% humidity. For feminized seeds or fast feminized / F1, you should know that once you switch your light to 12/12, your plant will enter flowering and it will double or triple in size! That's already a good indicator. Now it's up to you to decide the height you want to achieve. I would wait at least 21 days.

Autos are USUALLY not dependent on your light cycle to transition into flowering. Most autos will begin to flower on their own, usually between weeks 3 and 6. If you are going 7 weeks or longer with out seeing signs of flowering, the ruderalis genes may be recessive and at this point you would switch to 12/12 to try and get them to flower. This situation though is very rare and usually a result of unstable genetics. I have grow autos under 24/0, 18/6 and 14/10. 24 hrs got biggest buds but 14 hours got best yield for wattage used.

Auto plants flower when they want to (they only normally have a short veg of 3-4 weeks) not by light schedule. You can keep running 18/6 thru flower with autos to give u bigger nugs 👍