CREATE OR REPLACE FUNCTION to_hms (i_days IN number)
   RETURN varchar2
IS
BEGIN
   RETURN TO_CHAR (TRUNC (i_days)) || ' days ' ||
		 TO_CHAR (TRUNC (SYSDATE) + MOD (i_days, 1), 'HH24:MI:SS');
END to_hms;
/

-- 1.2345 days
SELECT to_hms (1.2345) FROM DUAL;
1 days 05:37:49

SELECT to_hms (SUM (x.dys))
  FROM (SELECT TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '10:00', 'dd-mon-yyyyhh24:mi')
               - TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '8:00', 'dd-mon-yyyyhh24:mi') dys
          FROM DUAL
        UNION ALL
        SELECT TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '13:00', 'dd-mon-yyyyhh24:mi')
               - TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '11:00', 'dd-mon-yyyyhh24:mi')
          FROM DUAL
        UNION ALL
        SELECT TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '16:00', 'dd-mon-yyyyhh24:mi')
               - TO_DATE (TO_CHAR (SYSDATE, 'dd-mon-yyyy') || '14:00', 'dd-mon-yyyyhh24:mi')
          FROM DUAL) x;

0 days 06:00:00