How to define "idle_time" ?

Check parameter of resource_limit
SQL> show parameter resource;

If it is 'False'
SQL> alter system set resource_limit = true;

Add it into init.ora file permanently
*.resource_limit=true;

Modify default profile
SQL> alter profile default limit idle_time 1440;
-- 1440 -> 24 hour

Check default profile setting
SQL> select profile, resource_name, limit from dba_profiles 
where lower(profile) = 'default' and resource_name = 'IDLE_TIME';

Monitor idle sessions
SQL> select * from v$session where status = 'SNIPED' order by logon_time desc;