SQL> declare
  2    ts timestamp := systimestamp;
  3    it interval day to second(3);
  4    ms integer;
  5  begin
  6    dbms_lock.sleep (3); -- wait 3 seconds;
  7    it := systimestamp - ts;
  8    dbms_output.put_line('Delay: '||it);
  9    ms := to_number(translate(substr(it,4),'0:.','0'));
 10    dbms_output.put_line('Delay (ms): '||ms);
 11  end;
 12  /
Delay: +00 00:00:03.000
Delay (ms): 3000

PL/SQL procedure successfully completed.

declare
v_ts1 TIMESTAMP;
v_ts2 TIMESTAMP; 
v_diff INTERVAL DAY TO SECOND;
v_diff_ms NUMBER;

BEGIN

v_ts1 := systimestamp;
Dbms_Lock.sleep(5.3);
v_ts2 := systimestamp;

Dbms_Output.put_line ( 'TS1 : ' || To_Char(v_ts1,'hh24:mi:ss ff6') ||  ' / TS2: ' || To_Char(v_ts2,'hh24:mi:ss ff6') );


v_diff := v_ts2 - v_ts1;
Dbms_Output.put_line ( 'Difference as Interval : ' || v_diff );

v_diff_ms := ( extract(hour from v_diff)   * 3600 +
               extract(minute from v_diff) * 60   +
               extract(second from v_diff) ) 
             * 1000; 
Dbms_Output.put_line ( 'Difference as MS     : ' || v_diff_ms );

END;

TS1 : 15:17:43 836901 / TS2: 15:17:49 137232
Difference as Interval : +00 00:00:05.300331
Difference as MS     : 5300.331