How long a page is kept

NimbleCache keeps a cached page until something changes it, and never longer than its Page lifetime, which is twelve hours by default. Saving a post, a price or stock change in WooCommerce, or saving a menu clears the pages each change affects at once; the lifetime is a safety net for a change nothing told the cache about.

Settings → Page lifetime offers an hour, six hours, twelve hours, a day, three days or a week. Front page lifetime can be shorter than the rest, which is worth doing on a site whose home page shows something that changes without anybody editing it. Left on The same as everything else, the front page follows Page lifetime.

When a page's lifetime runs out, the next visitor is sent a freshly built page, and that copy is stored for everybody after them. Visitors who arrive while it is being built do not each make WordPress build it again: they are sent the copy that has just expired, marked STALE in the X-NimbleCache header, until the new one is ready.

With Enable preloading on, pages are fetched again in the background, so nobody has to be the first visitor to a cold page. The Settings screen says so when a full pass over your pages takes longer than the page lifetime.