Jaxxxom, you are asking a question that is very difficult to answer. I will explain what i do and why you can do it differently.

I like low plant numbers and don't mind if it takes an extra 2 weeks of veg to have very few plants. Just as an example, I have a 3x4 space and I would use a single plant to fill the entire thing. I would allow the plant to fill the screen to about 80% full and then switch to 12 hour. This is the tricky part, you have to choose how tall you want your buds to be up from the screen based on how much light penetration you will get.
So if you are growing under a:

100W HPS/MH, you would want the buds to be about 3-4 inches tall when they are done. So you push them under the screen until you think they will get 3-4 inches taller and then quit.

400W HPS/MH, you would want the buds to be about 6 inches tall when they are done.

1000W HPS/MH, you would want the buds to be about 12 inches tall when they are done.

Basically, you have to consider how far into the "canopy" the light will penetrate and try to guess your buds to that tall. The first time you SCROG a strain this is kind of tough and i have a tendancy to make them too short instead of too tall because i just get into pushing them under and I love an even canopy.

So, there is no exact time for you to quit pushing them under. If you are growing aero and with a 1000W light for strain x, it is going to be completely different then growing DWC under a 400W light for strain y. I hope that makes sense.

No matter what you do it will be great. SCROG is awesome and it just takes once to hook you.

ask if you have questions.

Good luck