I found below statement and the link to the entire Congressional testimony from July 10, 2003 about how the FBI name check system works.
"Approximately 85% of name checks are electronically returned as having "No Record" within 72 hours. A "No Record" indicates that the FBI's Central Records System contains no identifiable information regarding to this individual. By agreement with State, partially due to our concern about the time factors in approving most visa requests, a No Record equates to a No Objection to the issuance of a visa."
http://www.fbi.gov/congress/congress03/garrity071003.htm
"Approximately 85% of name checks are electronically returned as having "No Record" within 72 hours. A "No Record" indicates that the FBI's Central Records System contains no identifiable information regarding to this individual. By agreement with State, partially due to our concern about the time factors in approving most visa requests, a No Record equates to a No Objection to the issuance of a visa."
http://www.fbi.gov/congress/congress03/garrity071003.htm