ISO体系认证证书有没有有效期有效期是多久？

<script> var _hmt = _hmt || []; (function() { var hm = document.createElement("script"); hm.src = "https://hm.baidu.com/hm.js?4c355d10edd5484d8a235161ce34a89b"; var s = document.getElementsByTagName("script")[0]; s.parentNode.insertBefore(hm, s); })(); </script> <!--商务通--> <script language="javascript" src="http://kht.zoosnet.net/JS/LsJS.aspx?siteid=KHT62834828&float=1&lng=cn"></script>