When to flip plant lighting from 18/6 to 12/12

If I were to grow bag seeds of an unknown origin how do you determine when to flip what are the signs you are looking for. I have only grown autos for the last year indoors and had only grown bag seeds till late 80s so it has been a very long time. Can I get some input from some veteran photo period growers.

Size of the grow space. A photo will stretch 2 to 3 times its height if you flip once it is mature.

What exactly do you mean I have a 4’x4’x80” tent and I do remember that between week 6 and week 12 to sex and remove males but how much longer do you go before you change from veg to flower. Can you explain the process to me? Start to finish I have read but want someone’s first hand knowledge.

I grow fems, so I skip the sexing part. Once a plant is vigorously vegging (around week 3 or 4), begins to alternate nodes, and maybe even show presex parts, it is ready to flower at any time from that point forward. From there you can continue to train and veg, but once you switch to 12/12 the plant will explode in growth. It will at least double in height (sativas even more) over the next 2 to 4 weeks. It will start needing more regular feeds, and if you are using veg nutrients you will want to switch over to your bloom nutrient.

I got some regular seeds with some autos I ordered and want to do a run of them soon but I believe I will have to weed out the males they were kinda like a bonus from the supplier which was DC Seed Exchange. Don’t want to waste those freebies. Plus I gave ground a few bag seeds from time to time and put them in bags marked them and stored them in a refrigerator for later use.

Males are pretty easy to spot, and in a 4x4 you have plenty of time to veg for several weeks and hope all of your plants show preflowers. You'll be able to sort them out, and you have at least a week after balls form to sort out any males before they pop most of the time! I say go for it.

Veg can be how ever long you want .. 2 -4 months depends how big you want them to be .. in a 5 gallon pot 1-1/2 months of veg is good . It all depends what you’re after . LST , topping can play into it …

Rate now I have 3gal fabric pots that I was using for the autos, but was thinking about going to a 5gal for photos. I thought the longer the grow that more root space would be needed so as to not get root bound.