WITH t_a AS
(
SELECT '09:00' stm, '13:00' etm FROM dual
)
, t_b AS
(
SELECT '09:30' tm FROM dual
UNION ALL SELECT '10:20' FROM dual
UNION ALL SELECT '12:00' FROM dual
)
SELECT COUNT(*) h
  FROM (SELECT ( TO_DATE(tm, 'hh24:mi')
               - TO_DATE(LAG(tm) OVER(ORDER BY tm), 'hh24:mi')
               ) * 24 h
          FROM (SELECT tm FROM t_a a, t_b b WHERE b.tm BETWEEN a.stm AND a.etm
                UNION ALL SELECT stm FROM t_a
                UNION ALL SELECT etm FROM t_a
                )
        )
 WHERE h >= 1
;

WITH T1 AS
(
SELECT '09:00' STIME, '13:00' ETIME FROM DUAL
)
, T2 AS
(SELECT '09:30' CHKTM FROM DUAL UNION ALL
SELECT '10:20' FROM DUAL UNION ALL
SELECT '12:00' FROM DUAL
)
SELECT SUM(CASE WHEN LEAST(TO_DATE(CHKTM,'HH24:MI'), TO_DATE(ETIME,'HH24:MI')) 
                   - GREATEST(TO_DATE(NVL(CHKFM, STIME),'HH24:MI'), TO_DATE(STIME,'HH24:MI')) >= 1/24 THEN 1 ELSE 0 END) 
     + MAX(CASE WHEN TO_DATE(ETIME,'HH24:MI') - TO_DATE(LASTTM,'HH24:MI') >= 1/24 THEN 1 ELSE 0 END)TM2
  FROM T1, 
      (SELECT CHKTM
            , LAG(CHKTM) OVER (ORDER BY CHKTM) CHKFM
            , MAX(CHKTM) OVER() LASTTM
         FROM T2)