// Auto Paging
function auto_paging($post) {
    global $page, $pages, $multipage, $more, $numpages;
    $content = $post->post_content;
    if (!strpos($content, '<!--nextpage-->')) {
        preg_match_all("#(<p[^>]*>(.*)</p>|<h[^>]*>(.*)</h(.*)>)#isU", $content, $paragraphs);
        $count = 1;
        $max_paragraph = 8;
        foreach($paragraphs[0] as $paragraph) {
            if ($count != $max_paragraph) {
                $count++;
                $temp_content = $temp_content.$paragraph;
            }
            else {
                $count = 1;
                $temp_content = $temp_content.$paragraph."<!--nextpage-->";
            }
        }
        $content = $temp_content;
        if( $page > 1 ) $more = 1;
        $multipage = 1;
        $content = str_replace("\n<!--nextpage-->\n", '<!--nextpage-->', $content);
        $content = str_replace("\n<!--nextpage-->",'<!--nextpage-->', $content);
        $content = str_replace("<!--nextpage-->\n",'<!--nextpage-->', $content);
        $pages = explode('<!--nextpage-->', $content);
        $numpages = count($pages);
        $post->post_content = $content;
    }
}
add_action( 'the_post', 'auto_paging' );