If user just closes the browser, system will log him out after 30 minutes. This is controlled by core platform in ow_system_plugins/base/bol/user_service.php:method - deleteExpiredOnlineUsers.
Since "offlining" users is connected to Cron, you will need to make sure that it is configured properly on your server.
But the problem is that if members dont loggout in my site they are showin still online. ( in profile and 'users' page)