I think Keefy has good logic about the Chamber and Slytherin, but it felt to me like the school was already standing when the Chamber was put inside it. In fact, it seemed like it was Slytherins inability to convince the others that allowing Muggleborns into the school is what prompted him to do so, right before he left. He was clearly at the school some time before that happened, although that could be as short a time as four years. The Lexicon has an essay on the Chamber here: http://hp-lexicon.org/hogwarts/castl...f_secrets.html
There's also this information, which came directly from Professor Binns in Chamber of Secrets:
"You all know, of course, that Hogwarts was founded over a thousand years ago - the precise date is uncertain - by the four greatest witches and wizards of the age. The four school Houses are named after them: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin. They built this castle together, far from prying Muggle eyes, for it was an age when magic was feared by common people, and witches and wizards suffered much persecution."
"For a few years, the founders worked in harmony together, seeking out youngsters who showed signs of magic and bringing them to the castle to be educated. But then disagreements sprang up between them. A rift began to grow between Slytherin and the others. Slytherin wished to be more selective about the students admitted to Hogwarts. He believed that magical learning should be kept within all-magic families. He disliked taking students of Muggle parentage, believing them to be untrustworthy. After a while, there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school."
"Reliable historical sources tell us this much. "But these honest facts have been obscured by the fanciful legend of the Chamber of Secrets. The story goes that Slytherin had built a
hidden chamber in the castle, of which the other founders knew nothing."
"Slytherin, according to the legend, sealed the Chamber of Secrets so that none would be able to open it until his own true heir arrived at the school. The heir alone would be able to unseal the Chamber of Secrets, unleash the horror within, and use it to purge the school of all who were unworthy to study magic."
However, another thing which needs to be pointed out: Slytherin and Gryffindor were the best of friends, so they had to have been less than ten years apart in age. Same thing for Ravenclaw and Hufflepuff.
Two excerpts from the Sorting Hat:
A thousand years or more ago
when I was newly sewn,
there lived four wizards of renown,
whose names are still well known:
bold Gryffindor, from wild moor,
fair Ravenclaw, from glen,
sweet Hufflepuff, from valley broad,
shrewd Slytherin, from fen.
They shared a wish, a hope, a dream,
they hatched a daring plan to educate young sorcerers
thus Hogwarts School began.
In times of old, when I was new,
And Hogwarts barely started,
The founders of our noble school
Thought never to be parted.
United by a common goal,
They had the selfsame yearning
To make the world's best magic school
And pass along their learning.
"Together we will build and teach"
The four good friends decided.
And never did they dream that they
Might some day be divided.
For were there such friends anywhere
As Slytherin and Gryffindor?
Unless it was the second pair
Of Hufflepuff and Ravenclaw, So how could it have gone so wrong?
How could such friendships fail?
Why, I was there, so I can tell
The whole sad, sorry tale.
Said Slytherin, "We'll teach just those
Whose ancestry's purest."
Said Ravenclaw, "We'll teach those whose
Intelligence is surest"
Said Gryffindor, "We'll teach all those
With brave deeds to their name."
Said Hufflepuff, "I'll teach the lot
And treat them just the same."
These differences caused little strife
When first they came to light.
For each of the four founders had
A house in which they might
Take only those they wanted, so,
For instance, Slytherin
Took only pure-blood wizards
Of great cunning just like him.
And only those of sharpest mind
Were taught by Ravenclaw
While the bravest and the boldest
Went to daring Gryffindor.
Good Hufflepuff, she took the rest
and taught them all she knew,
Thus, the Houses and their founders
Maintained friendships firm and true.
So Hogwarts worked in harmony
for several happy years,
but then discord crept among us
feeding on our faults and fears.
The Houses that, like pillars four
had once held up our school
now turned upon each other and
divided, sought to rule.
And for a while it seemed the school
must meet an early end.
what with dueling and with fighting
and the clash of friend on friend.
And at last there came a morning
when old Slytherin departed
and though the fighting then died out
he left us quite downhearted.
My personal feeling is that Slytherin and Gryffindor had to have been in their 50's-60's when the school was founded, and Ravenclaw and Hufflepuff probably at least 30, although certainly not as old as 50. The thing that even allows me to go so low in the first place is Helena Ravenclaw, who apparently attended Hogwarts, so the school was clearly running by the time she was seventeen and having her relationship with the Bloody Baron.
Very good use of canon sources and clearly presented. 5 points.