// define the desired amount of characters
$numberOfChars = 140;

// call the function
the_excerpt_max_charlength($numberOfChars);

// the function - copy this to the functions.php-file in your theme for example
function the_excerpt_max_charlength($charlength) {
   $excerpt = get_the_excerpt();
   $charlength++;
   if(strlen($excerpt)>$charlength) {
       $subex = substr($excerpt,0,$charlength-5);
       $exwords = explode(" ",$subex);
       $excut = -(strlen($exwords[count($exwords)-1]));
       if($excut<0) {
            echo substr($subex,0,$excut);
       } else {
       	    echo $subex;
       }
       echo "[...]";
   } else {
	   echo $excerpt;
   }
}

With a slight differentiation this function can not only be used for excerpt, but for a very general way to limit the words of a PHP string by specifying a maximum amount of characters. This is the function for general use:

// Shorten a String pattern to a maximum of characters without breaking words, by giving a String, maximum length and closing pattern if true
function cis_limit_words($pattern,$charlength=200,$after=" [...]") {
   $charlength++;
   if(strlen($pattern)>$charlength) {
       $subex = substr($pattern,0,$charlength-5);
       $exwords = explode(" ",$subex);
       $excut = -(strlen($exwords[count($exwords)-1]));
       if($excut<0) {
            echo substr($subex,0,$excut);
       } else {
            echo $subex;
       }
       echo $after;
   } else {
    echo $pattern;
   }
}

Because it proved to be useful I also provided this to the codex, improvements are welcome.