I hope no threads already exist for this, I couldn't find any.

Anyway, for the mirror of Erised, did Dumbledore mention that the mirror has been in that room for very long? It has been in Hogwarts for quite a while, and in Pottermore we find out some more info about it as well from JKR, (Not saying just incase people would rather wait to find out.) but

Is there any evidence on how long it remained in that disused classroom?

About how many children do you think found it?

Do you think all of the teachers knew of it's existence there?