Autos will transition around week 5/6 typically ( day 35 or so ) ....
To give it the BEST VEG would be at a higher cycle so it can push root mass .
Thus 24/0 or 20/4 .... however 18/6 will be fine .
Autos CAN grow under 12/12 ... however it can sometimes keep things smaller sometimes.
However Sativa Dominate cuts will flower nicely ....
Autos love light .... so prepping the plant for the MOST GROWTH DURING ITS SHORT VEG is a given. The week it transitions ( on its own ) it will start sexing ( pistils ) and preflower ( bud sites ) ...
I grew 24/0 and compared results with 18/6 . To me the difference is negligible.
Plants will show LIGHT SATURATION sometimes ( and look droopy ) as it slows down in photosynthesis.... there is a big bag of debate shit when it comes to whether plants NEED dark time.
I don’t change much from regular ( more natural light cycles as found in nature ) meaning probably the only place on the planet where 24 hours happen is Finland .... So why bother other than supplementing the veg growth.
Autos are fickle little shits too , when it comes to feed.
You can plant an auto in JUST GOOD POTTING SOIL and carry it up to transition time ( week 5 )
with just water. Then after it stops / slows stretch , I begin bloom.
One final ... most important of all .... FORGET BREEDER ESTIMATES ON HARVEST TIME.
Most auto growers will agree the plants perform much better when “ run til its done “ .
Most will deliver under 90 days , with a few running a bit longer ( sativas )