How are you going to use it? if it's just for support, you need it maybe 3 weeks into flower? Around the time it's vertical height is about half as tall as it will get. Some scrogs are used for training too, and that would be needed as soon as you want to level out the canopy.
my first scrog helps organize and stops vertical growth for 7-10 days - which helps keep the plant shorter in a 80" tall tent. It is 5-6" above the rim of my pots. I have irrigation drippers, so i don't have to worry too much about access to the top soil - as long as i can still top-dress seom mosquito bits, it's good enough. If you hand water, that's may require a bit more space for access. I put a 2nd one down a few weeks later that is just for support in case any of them can't hold their own weight late in flower.