"how tall could it grow? It would be a beast!"

Sounds good, but, not happening.

The tallest I've seen was some 20' hemp in Nebraska, followed by 18' dank in Hawaii.
The problem is senescence.
Weed is an annual.

Momma plants can be kept for quite a while, but they all, eventually, flower and die.
I have kept one under 24 hour light and it still went alternate, meaning it's ready to flower, after several months.
So, it's fun to think about, but mother nature runs this game.
There is more going on that just photoperiodicity.

I agree with Random999.
Huge trees are a pain in the butt.
 But with our limits of 3 mature gals per patient, we are forced to grow big here.

Aloha,
Weezard