Setting the limit from the_content figures

I am by using this code to create the limit of 300 figures towards thecontent material.

How do i set when i wish to utilize it so 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 wordpress

<?php

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

?>

Will shorten this content to 300 figures unless of course on one publish page.

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