When do you know you got to terminal lenght?

Welcome LHG. The best way to determine if your hair is at or near terminal length is to see if it is still getting longer. If your hair only seems to gain an inch of length in the time span of 6 months or a year, you are nearly at terminal length.
Also, if your hair is very thin at the ends, but thick farther up, you are at or near terminal length. You can see in my photo that my hair is quite thick at my shoulders, somewhat less thick at my waist, and thin and wispy close to my knees.
Uneven length is also another indicator. Take a good long look and you will see my hair is about 2 inches longer on the right side.
I hope this will clarify things for you. Long hair forever.
Absalom