Set excerpt length on the fly in wordpress.

wp-set-excerpt-length.php

<?php

/**
 * Sets the excerpt length for the output of the next excerpt
 *
 * @param int $length the number of words to show
 *
 * @return void
 */
function set_excerpt_length( $length = 50 ) {
	add_filter( 'excerpt_length', create_function( '$l', 'return ' . intval( $length ) . ';' ), 13 );
	add_filter( 'the_excerpt', create_function( '$e', 'remove_all_filters( "excerpt_length", 13 ); return $e;' ), 13 );
}

?>