设置the_content字符的限制

I'm using this code to set the limit of 300 characters to the_content.

How can i set when i want to use it and when i dont ?

<?php   
add_filter("the_content", "plugin_myContentFilter");   
function plugin_myContentFilter($content)
{
// Take the existing content and return a subset of it
return substr($content, 0, 300);
}
?>

<?php

    function plugin_myContentFilter($content) {
        if (!is_single()) {
            return substr($content, 0, 300);
        } else {
            return $content;
        }
    }
    add_filter("the_content", "plugin_myContentFilter");

?>

Will shorten the content to 300 characters unless on a single post page.