X 2. There were many posts last spring where people thought they had bought Leghorn pullets but mistakenly got Cornish X. Can you call the store to confirm they are "Cornish Rocks" and not white Rocks?
Are you sure they are Cornish X? Where did you get them?
If they are, they can survive for longer periods of time (usually about a year) but will need strict feed management. They won't be great layers. They generally come straight run so you probably have some males.