I had the same 'do as the kid from 3rd Rock From the Sun when I was an HS senior.
Longest? Past the shoulders.
Male pattern baldness set in and now I’m strictly a #2.
Mine was about down to my waist in my early twenties. I cut it after I started having kids. They seemed to delight in pulling it.
Mine, at it’s longest, hit my waist. It was too much to deal with. It’s still pretty long (about 3 inches above my waist when straightened–it’s curly naturally). After the wedding, I’m cutting it.
For most of my life I kept it shoulder length to halfway down my back, but lately I’ve been cutting it short. It’s all right in the winter but drives me crazy in the summer when I want to tie it all back, so I’m in the process of growing it back out again.
In high school, it was long, thick, and pin-straight, parted in the middle, at its longest waist-length. Partly because it was the style, partly out of sheer laziness, it didn’t need much maintenance except for occasional clipping of the ratty split ends (I hate seeing ratty split ends on otherwise pretty long hair!) - but it used to get all knotted up underneath at the nape of my neck and I’d have to carefully pick it apart strand by strand. I had two options to keep it under some kind of control: ponytail or loose braids. Neither looked very good, or professional, in my job, and long and loose - well, it just got too annoying and I finally took the plunge and got it cut to shoulder length.
I went to my high school reunion last month, and three different people looked at me and said “Sali? Is that Sali? What happened to your beautiful long hair???” Made me feel like I had been Cousin It from the Addams Family, scuttling along in the halls…
I once had mine all the way down to my rear. I’m a gal. That was in high school.
Now I’m 44 and it’s shoulder length.
My hair is 30.5 inches long, it is almost to my tailbone. It is longer than it has ever been. I’ve been using the tips and tricks I’ve found at longhaircommunity.com to take better care of my type of hair. My hair breaks a lot and through advice there, my hair is better than it has ever been.
Now that I broke the bra strap barrier, I can actually wonder how long I want it.
When I was eleven, my hair came down to the bottom of my shoulder blades. Then it went curly, and it now hovers around shoulder length.
I’m 50 years old and my hair is 50 centimeters (~20 inches). That’s bra-strap length, middle of the back. This is the longest I’ve had it, and it’s been this length any number of times over the years. Like water, it just seems to seek its own level.
This is the longest my hair has ever been - that photo was taken in the winter of 2007-2008. That’s my sister there, and in the dressing room behind us, my dad.
A lot of people seemed to really like this look and when I cut my hair short and shaved my face, dozens of hearts were broken.
I checked mid-back, but as a curly headed guy, I could pull it straight until it touched my ass.
Wooda been more interesting to have broken this down by gender.
And then there’s this guy:
We used to have a test to determine if your hair was really long or not: When standing in the shower, let the water run down your back, soaking your hair and pulling it to its full length. Then, while keeping your head level, clench your butt cheeks. If you then cannot tilt your head down, then you had what we referred to as “proud length”.
I’ve had hair down past my back a couple of times - I, er, suppose that I would have passed Hal Briston’s test. The last time was about ten years ago. It just became too much trouble - my SO once woke me up in the middle of the night because my braid had looped itself around his neck. 
When it’s wet my hair is about halfway down my back but it sproings up into curls around my shoulders now. A few years ago it hit my tailbone. I was in construction then so I wore it in braids most of the time.
Longest ever would be mid back. It’s between my midback and shoulders right now. If it wern’t for this little modern marvel, it’d be much shorter, because I’d wash it every day.
Dreadlocks - most hit the back of my knees. Recently celebrated the 25th anniversary of my last haircut.
Back of your knees? I think you win. Picture?
I’ve had long straight hair (mid-back or longer) for almost all of my life. At it’s longest I could just barely sit on it. When I was 18 I had it cut from waist length up to my chin… and I’ll never do that again. When it was that short I looked like a high school freshman instead of college.
Now I’m 24 and my hair is about 2" above my hip bones. It is annoying sometimes and I often have moments of wanting to cut my hair but I just haven’t found a style that suits me well without making me look too young.