The length of time that a client browser, upstream device, or BIG-IP keeps compiled content in its cache before refreshing it is called content lifetime. Content lifetime is expressed in the form of a time to live (TTL) value, and can vary for each cached response.
When content is in cache longer than its TTL value, the BIG-IP considers the content expired. When the BIG-IP receives a request for expired content, it sends that request to the origin web servers for fresh content, replaces the expired cached content with the fresh response, and then responds to the request.
You change your site's content approximately every 4 hours. You want the BIG-IP to cache content for no longer than 24 hours. If the origin web servers are not responding for request for fresh content, you are willing to allow the BIG-IP to serve content that is 8 hours old (or twice the age of the content).
To ensure that you can manage content invalidation, you do not want to rely solely on the browser's local cache settings for the home node, so you do not have a minimum time set for content residing in the browser cache before performing a check for content freshness.
The content served for your general applications changes about once every 4 hours. You use an invalidations rule to force a refresh when content changes, but you do not want content to remain in the system's cache for more than 5 hours without a refresh.
If the origin web servers are not responding to the BIG-IP's refresh requests, you are willing to allow the BIG-IP to serve content that is 8 hours old (or twice the age of the content).