https://www.pcisecuritystandards.org/documents/pci_dss_v2.pd...
See section 8.5.9 for the password change policy. Section 3.6.4 for the key rollover policy (which references NIST 800-57 that basically says 1-3 years for most keys.
There's other standards (such as the 1-5 years for an SSL certificate from the issuer) and tons of best practices out there. In short you should be rolling over keys and expiring passwords on some sort of schedule. My point was that because humans use passwords they are conventionally seen as needing quicker expiration than keys.