Allow files of up to a certain song length

I have a very limited knowledge of Module design so i really don't think i will be the best person to implement it. I have tried to implement another module which on the form validation allows an mp3 up to a certain length but when enabled it stops the user logging in.

Maybe you can give me an idea of what i am doing wrong?

<?php
// $Id$
/**
* @file
* Simple module to run some custom validation on a story node
*/
 
/*
* Implementation of hook_form_alter()
*
*/
function tise_helper_form_alter(&$form, $form_state, $form_id) {

$form['#validate'] = array('_tise_helper_new_post_form_validate');

return $form;
}


 
/*
* Validates the story node form, and throws an error to the user if validation fails
*
*/
function _tise_helper_new_post_form_validate($form, &$form_state) {

$long = $form_state['values']['field_post_audio_upload'][0]['data']['duration'];

if($long > 11.0)
{
form_set_error('field_post_audio_upload', t('Song Length must be less than 10 seconds! Your song is '.$long.' seconds long'));
}
 
}