The disk cache TTL (Time To Live) specified in hours.

When the disk cache of a page is generated it is time-stamped. This copy will only be used until it expires. The amount of time before it expires is set via this TTL. If someone visits a page that uses caching after the cached copy has expired, a fresh copy will automatically be generated.

A separate TTL is available for users and for guests (typically if they are not kept the same, the guest TTL will be longer then the user TTL)

When considering how long to set the TTL consider that pages receiving little traffic will not benefit from caching if the TTL is set too low (as cached copies will expire before subsequent visitors can view the page and receive the cached copy) for example if the TTL is set to 1 hour and the page, on average only receives one visit every hours then the cache is effectively useless. Should the TTL for that same page be set to 12 hours then about 11 of those visits will benefit from the cache (the 12th will not because a new copy will need to be rendered)

The default value for TTL is 12 hours.