We’re discussing hair arising from the scalp, right?
Personally, long hair to me is shoulder length and longer. Really, it is any length where it is often “put up” to keep it out of the way during strenuous activities.
Anything past the armpit is really long. And really attractive in my opinion. But not many women have the time/inclination/hair strength to wear hair that long.
To me, it’s bra-strap length, or mid-back. Currently mine’s about waist length and I’m thinking of cutting it, but I don’t want to go any shorter than that, or I won’t feel like a longhair anymore.
If armpit hair reaches the shoulders or mid-back, yes, I think that qualifies as long.
For me and my hair, shoulder length is long. The longest I could ever grow it was about 2" past my shoulders. 
But for the purposes of this discussion, I’d call armpit length long.
I had a friend whose hair reached the floor - I call that insanity! Altho Locks of Love (or one of those groups) did benefit when she decided to cut it off.
The authorities have spoken:
I voted “shoulder length”, because I thought my hair was long when it was down to my shoulders. (I am a guy, but standards are standards).
That’s for straight hair, which is what I got. Currently more or less a buzz cut. For curly hair, I don’t know what my standard would be. The Lovely and Talented Mrs. Shodan has curly hair, and it would be shoulder-length if it were straight. But I don’t think of her as having long hair.
FWIW.
Regards,
Shodan
I said shoulder length but that would be assuming all the various lengths would have to fall into either the short or long category.
I said armpit length, though I really meant “longer than shoulder length”. If it’s just brushing the shoulders it’s not quite long enough for me to see it as “long”.
Not necessarily. 
Shoulder-length is base-level for women’s hair. It not short nor long. Armpit lenght is were long begins.
I agree. Long hair starts when it gets below the shoulder.
Ditto.
Touching the ear or collar equals long. 
Actually, off the shoulder equals medium. Past the shoulder equals long.
Context is everything.
For a woman with shoulder-length hair, if someone asks “does she have long hair or short hair”, I would definitely say “long”.
In a police BOLO or APB, the description for a woman with shoulder-length hair would probably be “…, long hair, …”. So even shoulder-length hair is long hair.
However, if I’m sitting with a group of friends discussing each other’s hair lengths, it would be said that those with mid-back-length hair have ‘long hair’, while those with shoulder-length hair would be ‘shorter’.