Modify the excerpt lenght (Wordpress default is 55 words)

functions.php

<?php
 //modify the excerpt lenght (Wordpress default is 55 words)
add_filter('excerpt_length', 'my_excerpt_length');
function my_excerpt_length($length) {
	if (is_archive()) {
		return 55; // This is the number of words in archive pages (e.g. category overview). 
    } else {
        return 55; // This is the number of words in all other excerpts
    }
}
?>