User is being Asked for Credentials every 30 Minutes

If you encountered a situation where a user is being asked for credentials every 30 minutes, then this is most likely caused by the fact that Kerberos is being used and SPN is set using only the FQDN.

MORE INFORMATION
If you set the SPN by using only the FQDN of the server that is running IIS, you will be prompted for your user credentials after 30 minutes. The 30-minute time-out occurs because of the way that Internet Explorer caches Domain Name System (DNS) information. After 30 minutes, Internet Explorer reverts to the NetBIOS name. Therefore, you must make sure that you also register the SPN by using the NetBIOS name of the server that is running IIS to avoid being prompted for your user credentials. For more information, click the following article number to view the article in the Microsoft Knowledge Base: 
263558 (http://support.microsoft.com/kb/263558/) How Internet Explorer uses the cache for DNS host entries.

Please see the full article here:
http://techrepublic.com.com/5208-6230-0 ... ID=2500161