/**
 * Implements hook_field_formatter_settings_summary().
 */
function {{ machine_name }}_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];

  $summary = '';

  if ($display['type'] == 'text_trimmed' || $display['type'] == 'text_summary_or_trimmed') {
    $summary = t('Length: @chars chars', array('@chars' => $settings['trim_length']));
  }

  return $summary;
}