On a single server system, I've got the following setup:
DB cache - memcached - 300 seconds cache object lifetime.
Page cache - disk enhanced - 300 seconds max lifetime and 300 seconds garbage collection.
Display a page and the disk copy is created. Display the page again and the disk copy is served to the browser, even to another browser. But after 5 minutes, I'm still getting the first cached file. A new copy is not created, even if I make a change to a post that renders on the page.
Once in a while, a new version is generated. This seems to be because the disk cache is flushed and so a new copy of the page will be cached.
I'm not sure where to look next.