I was asked recently about caching expiration policies in response to my rant in my Caching should not be the source of truth post and a comment I made in a post by Chris Blankenship. I have written an article about cache expiration policies which you can find here. It discusses the high level concepts in expiration polices and some suggestions about which options to pick. It makes references to HttpRuntime.Cache and the Caching Application Block in EntLib.