Oracle 11g Release 2

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CREATE TABLE tv_max ( tv_uid, r_timestamp_arr ) AS
SELECT '7a87e8e4861a4d0aae65da1a7248b256', DATE '2019-12-27' + INTERVAL '00:00' HOUR TO MINUTE FROM DUAL UNION ALL
SELECT '7a87e8e4861a4d0aae65da1a7248b256', DATE '2019-12-27' + INTERVAL '01:30' HOUR TO MINUTE FROM DUAL;
2 rows affected

DECLARE
v_rep number;
BEGIN
  SELECT to_number(
           REPLACE(
             max(
               substr(to_char(r_timestamp_arr,'HH24:MI'),1,2)
               ||ltrim(to_char(round(to_number(Substr(to_char(r_timestamp_arr,'HH24:MI'),4,2))/60,2),'.00'))
             ),
             '.',
             ','
           )
         ) 
         -
         to_number(
           REPLACE(
             MIN(
               substr(to_char(r_timestamp_arr,'HH24:MI'),1,2)
               ||ltrim(to_char(round(to_number(Substr(to_char(r_timestamp_arr,'HH24:MI'),4,2))/60,2),'.00'))
             ),
             '.',
             ','
           )
         )
  INTO   v_rep
  from   TV_MAX
  where  TV_UID = '7a87e8e4861a4d0aae65da1a7248b256';
  
  DBMS_OUTPUT.PUT_LINE( v_rep );
end;
/
ORA-01722: invalid number
ORA-06512: at line 4

DECLARE
  v_rep NUMBER;
BEGIN
  SELECT ( MAX( r_timestamp_arr - TRUNC( r_timestamp_arr ) )
         - MIN( r_timestamp_arr - TRUNC( r_timestamp_arr ) )
         ) * 24
  INTO   v_rep
  FROM   tv_max
  WHERE  TV_UID = '7a87e8e4861a4d0aae65da1a7248b256';

  DBMS_OUTPUT.PUT_LINE( v_rep );
END;
/
1 rows affected

dbms_output:
1.5

DECLARE
  v_rep NUMBER;
BEGIN
  SELECT ( MAX( r_timestamp_arr ) - MIN( r_timestamp_arr ) ) * 24
  INTO   v_rep
  FROM   tv_max
  WHERE  TV_UID = '7a87e8e4861a4d0aae65da1a7248b256';

  DBMS_OUTPUT.PUT_LINE( v_rep );
END;
/
1 rows affected

dbms_output:
1.5