Three-fifths
Here's the specific statement from within the Illinois Constitution (Article IV, Section 9): The house to which a bill is returned shall immediately enter the Governor's objections upon its journal. If within 15 calendar days after such entry that house by a record vote of three-fifths of the members elected passes the bill, it shall be delivered immediately to the second house. If within 15 calendar days after such delivery the second house by a record vote of three-fifths of the members elected passes the bill, it shall become law.