Founders era timeline
So, we know that at some point Salazar Slytherin leaves the other Founders because he's angry about Muggle-borns being allowed to attend. Some time before this, Godric Gryffindor used his hat to create the Sorting Hat so that after the Founders were all dead, the four houses would still exist - as I recall, Gryffindor wanted to end Slytherin House, but couldn't because the Hat refused to Sort students any differently.
We also know that Helena Ravenclaw was a member of her mother's house, and at some point after graduation stole Rowena's diadem and went to Eastern Europe. As Rowena was dying, she asked the Bloody Baron to go and get Helena back, but he ended up killing them both.
So my question is, do you think Helena Ravenclaw would have known Salazar Slytherin before he left? How long do you think Slytherin was at Hogwarts before leaving?