Yes, the longer you veg the bigger the plant. 8sq ft for 1 plant is a lot of room. It will take considerably more than 40 days to fill it. Look at Topping, SCROG, LST, Supercrop etc. for maximum use of your space.
Good dirt is a must, I recommend something that says something like "professional growing substrate".
When to flip? These pics were taken 8 days apart, I had just fliped to 12/12 in the secon pic.
The 6 plants are now taking up 4sq ft each, the tallest is 4ft, and the canopy is at just over 3ft.
Have fun