You can add it straight to the template before the header in most cases...

<?php /* Template Name: Products */ ?>
<?php 
  add_filter( \'excerpt_length\', function( $length ) {  return 10; }, 999);
  get_header(); 
?>

Method 2: In the loop

$slug = get_page_template_slug($post->ID);
if(\'page-products.php\' == $slug) {
        add_filter( \'excerpt_length\', function( $length ) { 
           return 20; 
        }, 999);    
}

if ( is_page_template( \'page-products.php\' ) ) {
    add_filter( \'excerpt_length\', function( $length ) { 
       return 20; 
    }, 999);
}
This could also be used to get the template....
global $post;
$template = get_post_meta($post->ID,\'_wp_page_template\',true);
Or possibly even this (untested):
add_action(\'get_header\', function() {
    if ( is_page( array( 37, \'Products\' ) ) ) {
        add_filter( \'excerpt_length\', function( $length ) { 
           return 20; 
        }, 999);
    }
}, 1);
Or as seen here, you can create your own function to allow the excerpt length to be determined on a need to need basis by the amount of characters.
function excerpt($limit) {
  $excerpt = explode(\' \', get_the_excerpt(), $limit);
  if (count($excerpt)>=$limit) {
    array_pop($excerpt);
    $excerpt = implode(" ",$excerpt).\' ...\';
  } else {
    $excerpt = implode(" ",$excerpt);
  } 
  $excerpt = preg_replace(\'`\\[[^\\]]*\\]`\',\'\',$excerpt);
  return $excerpt;
}
 要显示的内容：
<?php echo excerpt(50); ?>