<?php
add_filter('the_content', 'hjs_insert_content_after_chars_words');
function hjs_insert_content_after_chars_words($content) {
  // only do this if post is longer than 1000 characters
  $enable_length = 1500;
  // insert after the first </p> after 500 characters
  $after_character = 1500;
  if (is_single() && strlen($content) > $enable_length) {
    $before_content = substr($content, 0, $after_character);
    $after_content = substr($content, $after_character);
    $after_content = explode('</p>', $after_content);
    $text = 'Add Content Here';
    array_splice($after_content, 1, 0, $text);
    $after_content = implode('</p>', $after_content);
    return $before_content . $after_content;
  }
  else {
    return $content;
  }
}