Well that is some bad information right there. I have been topping autos for at least 3 years now and
@autobeast has a great thread to teach you all about it
https://www.autoflower.org/threads/auto-topping-thread-pushing-the-auto-limits.57281/
The only issue you will have with the HID lamp is if you get too close and burn them up. Hang the light as high as you can to prevent that. then as they get older use the back of the hand method to set the height. If the back of your hand gets hot at the canopy level the light is too close. Do not use plastic cups to cover the seedlings they will get too wet and become susceptible to damping off. Keep temperature ~78°F with 60% RH.
As to the length of day, I have run 24/0, 20/4 and 18/6. 18/6 turns out to be the optimal inputs vs return ratio. Yes, I got a little bit bigger yield with 20/4 but when I calculated the energy used to run the extra hours it was a loser. There was no difference in yield between 24/0 vs 20/4 in my test grows.
You keep asking how long it takes to grow an auto; well that is like asking how long is a rope! It depends on way too many variables. Here is my rule of thumb when I plant autos vs photos (I run photos in the summertime here they handle the heat better.) I plan 90 days for autos and 120 to 150 for photos. I ran Wild Thailand a couple of summers back and they took almost 180 days but they were worth the wait.
I grow 6 plants in a 4x6x8 foot space. I highly recommend Fast Buds Green Crack Autos for a first grow here is a picture of the last time I grew them:
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I know nothing about their selection. Wonder what I should pick.. Any recommendations are welcome lol
