SQL> select * from t order by action_time;
ACTION_NAME ACTION_TIME
----------- -------------------
logon       20/09/2019 16:35:38
logoff      20/09/2019 16:56:07
logon       20/09/2019 17:46:38
logoff      20/09/2019 20:54:31
logon       20/09/2019 22:24:02
logoff      20/09/2019 23:27:09
logon       21/09/2019 03:57:56
logoff      21/09/2019 05:22:19
logon       21/09/2019 05:27:31
logoff      21/09/2019 06:02:14

10 rows selected.

SQL> with
  2    durations as (
  3      select action_name,
  4             action_time-lag(action_time) over (order by action_time) duration
  5      from t
  6    )
  7  select sum(duration) days,
  8         to_char(trunc(sysdate)+sum(duration),'HH24:MI:SS') hours
  9  from durations
 10  where action_name = 'logoff'
 11  /
      DAYS HOURS
---------- --------
.271238426 06:30:35

1 row selected.

create table USERS_ACTIVITY
(
  USERNAME    VARCHAR2(30),
  TERMINAL    VARCHAR2(30),
  SESSIONID   NUMBER,
  OS_USERNAME VARCHAR2(30),
  ACTION_NAME VARCHAR2(25),
  TIME_IN_OUT VARCHAR2(50)
);
-------------------------------------------------------
entry through forms 6i.
---------------------------------------------------
declare
	username	varchar2(30);
	terminal	varchar2(30);
	sessionid	number;
	os_username	varchar2(30);
	action_name	varchar2(25);
	time_in_out	date;

begin
select a.USERNAME,a.TERMINAL, a.sessionid, a.OS_USERNAME,ACTION_NAME,to_char(timestamp+(1/24*5),'dd/mm/yy hh:mi:ss')

into username,terminal,sessionid,os_username,action_name,time_in_out from dba_audit_trail a
where a.sessionid=(select max(sessionid)from dba_audit_trail where username=user);
insert into users_activity
(
username,terminal,sessionid,os_username,action_name,time_in_out
)
values
(username,terminal,sessionid,os_username,action_name,time_in_out);
end;
commit_form;

action_name logon/logoff Time
----------- ------------------
logon        19/09/19 10:00:00
logoff       19/09/19 11:00:00
logon        19/09/19 13:00:00
logoff       19/09/19 14:00:00
-------------------------------------
------------------------------------
Total Duration: 02:00:00 (hours)

with 
  durations as (
    select action_name, 
           TIME_IN_OUT-lag(TIME_IN_OUT) over (order by TIME_IN_OUT) duration
    from USERS_ACTIVITY
    WHERE USERNAME = <USERNAME>
  ) 
select sum(duration) days,
       to_char(trunc(sysdate)+sum(duration),'HH24:MI:SS') hours
from durations 
where action_name = 'logoff'
/

SQL> select action_name,
  2         to_date(TIME_IN_OUT,'DD/MM/YY HH24:MI:SS') TIME_IN_OUT
  3  from USERS_ACTIVITY
  4  where USERNAME = 'HIJAB'
  5  /
ACTION_NAME TIME_IN_OUT
----------- -------------------
LOGON       23/09/2019 10:14:04
LOGOUT      23/09/2019 10:59:04
LOGON       23/09/2019 02:16:26
LOGOUT      23/09/2019 03:16:26

4 rows selected.

SQL> with
  2    data as (
  3       select action_name,
  4              to_date(TIME_IN_OUT,'DD/MM/YY HH24:MI:SS') TIME_IN_OUT
  5       from USERS_ACTIVITY
  6       where USERNAME = 'HIJAB'
  7    ),
  8    durations as (
  9      select action_name,
 10             TIME_IN_OUT-lag(TIME_IN_OUT) over (order by TIME_IN_OUT) duration
 11      from data
 12    )
 13  select sum(duration) days,
 14         to_char(trunc(sysdate)+sum(duration),'HH24:MI:SS') hours
 15  from durations
 16  where action_name = 'LOGOUT'
 17  /
      DAYS HOURS
---------- --------
.072916667 01:45:00

1 row selected.