Dynamically set configuration on session expire doesn't work

[eluser]Usman -Halal IT[/eluser]
Hi,
I have created an authentication library which uses CodeIgniter's session class. By default the session cookie remains alive for 7200 seconds as set on config.php but I want to increase it to 30 days if users check Remember Me in login form.

Code:

if($this->input->post('remember'))
{
    $this->config->set_item('sess_expiration', '2592000');
}

It sets successfully but it expires very soon. Please give me any suggestions.

Thanks,
Usman.