Found this section. It does not seem to differentiate between Domestic and International.
First Class International shows no estimate.

Code:
      /********************* CUSTOM START:  IF YOU HAVE CUSTOM TRANSIT TIMES ENTER THEM HERE ***************/
      switch (TRUE) {
        case (preg_match('#Priority Mail Express#i', $service)):
            $time = '1 - 2 ' . MODULE_SHIPPING_USPS_TEXT_DAYS;
          break;
        case (preg_match('#Priority MailTM#i', $service)):
            $time = '2 - 3 ' . MODULE_SHIPPING_USPS_TEXT_DAYS;
          break;
        case (preg_match('#Standard PostRM#i', $service)):
            $time = '4 - 7 ' . MODULE_SHIPPING_USPS_TEXT_DAYS;
          break;
        case (preg_match('#First\-Class#i', $service)):
          $time = '2 - 5 ' . MODULE_SHIPPING_USPS_TEXT_DAYS;
          break;
        case (preg_match('#Media MailRM#i', $service)):
        default:
          $time = '';
      /********************* CUSTOM END:  IF YOU HAVE CUSTOM TRANSIT TIMES ENTER THEM HERE ***************/