It doesn't seem like WordPress has such function out of the box that returns the length of a post (number of characters or words).
You can use the Post Word Count plugin to get the number of words per post, but you will have to modified the plugin first.
Open post-word-count.php, replace echo number_format($totalcount); with return $totalcount;
Install and activate the plugin.
Open your theme's sidebar.php, add the following code to wherever you want to second ad to show up.
<?php
if(mdv_post_word_count(true) >= 100){ //replace 100 with the desire number of words.
?>
YOUR AD CODE HERE
<?
}
<?>