Apache Reference: mod_proxy, CacheGcInterval
Interval Between Proxy Cache Garbage Collections
Syntax: CacheGcInterval A
Example: CacheGcInterval 48
Since: Apache 1.1
This directive instructs the proxy server to check the document cache
at the interval specified in the hours argument, and to delete files
if the space usage exceeds that set by CacheSize. Note that the hours
argument can contain float value.For example, you can use ``1.5'' to
check the cache every 90 minutes. Keep in mind that, by default, no garbage collection
is performed and the cache will grow indefinitely. The
larger the hours argument, the more extra space beyond the configured
CacheSize that will be needed for the cache between garbage collections.