Session Fixation and CodeIgniter Session Libary

[eluser]searain[/eluser]
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 900;
$config['sess_expire_on_close'] = TRUE;
$config['sess_encrypt_cookie'] = TRUE;
$config['sess_use_database'] = TRUE;
$config['sess_table_name'] = 'ci_sessions';
$config['sess_match_ip'] = FALSE;
$config['sess_match_useragent'] = TRUE;
$config['sess_time_to_update'] = 300;

Thanks.

$config['sess_time_to_update'] = 300; is the set up interval for rotating the session id? For a high security required site, what value should I set up?

Thanks!