Apache Reference: mod_proxy, CacheDefaultExpire
Default Expiration Time for Cached Documents
Syntax: CacheDefaultExpire A
Example: CacheDefaultExpire 6
Since: Apache 1.1
This directive sets the default time in hours for which the proxy server caches
a document. If the document is fetched via a protocol that does not
support expiration times (for instance, FTP), then the value in the hours
argument is used as the expiration time. CacheMaxExpire does not override this
setting.