autocomplete.limit.js

(function($) {

  /**
   * Keep an unmodified copy of the core search callback function.
   */
  Drupal.ACDB.prototype.coreSearch = Drupal.ACDB.prototype.search;

  /**
   * Override the search callback.
   *
   * Prevent autocomplete search from running if the search string is shorter
   * than the limit. Otherwise, call the clone of the original search callback.
   */
  Drupal.ACDB.prototype.search = function (searchString) {
    var limit = Drupal.settings.autocomplete_limit['limit'];

    // Do not search if the string is too short.
    if (limit && searchString.length < limit) {
      return;
    }

    // Call the original core search callback.
    return this.coreSearch(searchString);
  };

  // Good night, and good luck.
})(jQuery);