Apache Reference: mod_proxy, CacheMaxExpire

CacheMaxExpire

Maximum Expiration Time for Cached Documents
Syntax: CacheMaxExpire A
Example: CacheMaxExpire 12
Since: Apache 1.1

This directive sets the maximum time in hours for which the proxy server caches a document. Cachable HTTP documents will be retained for at most hours without checking the origin server. Thus they can be at most hours out of date. This restriction is enforced even if the document has its own expiration time.