I've been asked what is terminal length for hair, here is what I have found:

Terminal or Maximum hair length

A person’s terminal or maximum length will be determined by several factors:
1.  Average growth per month
2.  Average hair loss/molting each day
3.  General health of the person
4.  Age
5.  Gender
6.  Hormonal balance (pregnancy, menopause)
7.  Hair type
8.  Hair condition

The average hair growth is ½ inch per month for women, a bit less for men.  People living in warmer climates might experience more rapid growth.  Genetics also play a role in monthly hair growth.  

The more hair you lose each day, the fewer hairs that are available to reach longer lengths.  This loss is referring to hair was shed from the scalp and not a broken hair.  The longer the hair stays attached to your head, the longer the hair can get.  When a hair is shed, that follicle will rest before producing a new hair.  The fewer shed hairs, the more hair you have on your head and the longer the hair itself can get.

Poor health, nutrition, stress, etc can weaken the hair follicle causing it either grow slower or fall out.

Women’s hair grows fastest between the ages of 16 and 30.

Hormone changes play a significant role in hair growth.  Puberty seems to be the start of a person’s most rapid hair growth. Pregnancy typically brings on rapid growth (with significant shedding after the birth).  Menopause frequently brings on slower growth and more shedding.

The more hairs you have on your head, the higher the likelihood your hair will grow longer.  

Hair in good condition is less likely to split or break, allowing it grow to its maximum length.

[Reference:  The Hair Grower's Handbook

I also strongly believe that how we handle and care for our hair can have a significant affect on how long our hair can grow, as well as how thick it can be.  Last night, I watched my sister comb her BSL length hair, using quick, strident, harsh strokes.  I almost winced.  I am extremely careful how I detangle.  She probably thinks I'm silly with how gentle I am with my hair.  I baby it!

Also included in proper long hair management is to minimize as best as possible what harsh products are used in your hair, such as avoiding perming/straightening, dyeing/coloring (especially bleaching), blow-drying, and curling irons, and also other poor handling such as brushing when wet, or brushing/combing from the scalp thru the ends.  For most long lengths, people should gently detangle from the ends up to the scalp (although once your hair grows beyond knee length, this takes on other problems, simply because the hair is longer than the arms, so you have to work your hair in horizontal sections, lifting it up -- it gets complicated!).

Simply put, to minimize hair loss and damage, i.e. to encourage growth and thickness, we have to protect our hair from ripping, breaking, yanking, and brittleness, and practicing the above habits will greatly cut down on hair loss due to those problems.

Jennifer Eve