how to destroy session automatically after defined time?

As my title said the question, i want to destroy my session after a particular time if user is being ideal..
currently i am using this code to define my session:

session_start();
// Check, if username session is NOT set then this page will jump to login page
if (!isset($_SESSION['myusername'])) {
header('Location: admin.php');
}

Code (markup):

 

http://stackoverflow.com/questions/520237/how-do-i-expire-a-php-session-after-30-minutes