<?php

//this part goes to functions.php

function string_limit_words($string, $word_limit)
{
  $words = explode(' ', $string, ($word_limit + 1));
  if(count($words) > $word_limit) {
  array_pop($words);
  //add a ... at last article when more than limit word count
  echo implode(' ', $words)."..."; } else {
  //otherwise
  echo implode(' ', $words); }
}

//this part goes to template

  $excerpt = get_the_excerpt();
  echo string_limit_words($excerpt,25);

?>

http://wordpress.org/support/topic/limit-the-number-of-words-in-excerpt-without-plugins

Wordpress excerpt custom word limitation

wordpress

PHP