/**
 * Implements hook_field_validate().
 */
function {{ machine_name }}_field_validate($entity_type, $entity, $field, $instance, $langcode, $items, &$errors) {
  foreach ($items as $delta => $item) {
    if (!empty($item['value'])) {
      if (!empty($field['settings']['max_length']) && drupal_strlen($item['value']) > $field['settings']['max_length']) {
        $errors[$field['field_name']][$langcode][$delta][] = array(
          'error' => 'text_max_length',
          'message' => t('%name: the value may not be longer than %max characters.', array('%name' => $instance['label'], '%max' => $field['settings']['max_length'])),
        );
      }
    }
  }
}