According to the Apple developer documentation, each distribution certificate only lasts for one year:
"A distribution certificate is valid for one year from date of issue." http://developer.apple.com/library/ios/#documentation/ToolsLanguages/Conceptual/DevPortalGuide/ManaginganiOSDistributionCertificate/ManaginganiOSDistributionCertificate.html
I just created a new distribution certificate today (in an Enterprise portal), and it seems to be valid for two years? Has Apple changed something recently?