Hello. I’m trying to write a story about the Founders, but there are a few things I’d like to get other opinions on.

My first question has to do with the actual founding of Hogwarts. How long would it have taken? Did the four friends just get together one day and decide they were going to start a school and then go right out and do it? Did they plan for it? Or was it more of a gradual idea that came on over time? This concerns me quite a bit because it plays into their ages – how old they would have been when the idea was formed, how long they worked on it, and how old they’d be by the time of my fanfiction.

I kind of feel like it would have taken a while to found the school, because a castle the size of Hogwarts doesn’t just spring up overnight. Unless they used an already existing castle that had been abandoned, and just fixed it up a bit?

My other question is about the Sorting Hat. When was it created? Any ideas? It’s song in Goblet of Fire says: While still alive they did divide/Their favorites from the throng/Yet how to pick the worthy ones/When they were dead and gone?/Twas Gryffindor who found the way/He whipped me off his head/The founders put some brains in me/So I could choose instead!

Do you think they did that before any of them died, or after? And would it have been before or after Slytherin left the school?

And, speaking of Slytherin, how much time do you think passed between the founding/opening of Hogwarts, and the time he left the school? According to the song in Order of the Phoenix:
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/Then at last there came a morning/When old Slytherin departed/And though the fighting then died out/He left us quite downhearted.
The only references to time here are “for several happy years” and then “for a while” before Slytherin left “at last.” We also know that Slytherin had something to do with the Chamber of Secrets, whether he built it himself or simply used a tunnel the others didn’t know about. What do you think? How long did it take Slytherin to become discontented enough with the other founders to leave the school?