Any general rules of thumb on WP Super Cache expire times?

Our expire time is 3600 seconds / 1 hour (not sure if that is default or I changed it) and am not understanding clearly why or if we should raise this time. Our sites don't get many comments, so the content isn't really changing and it seems like caching pages at longer intervals will use less resources. After reading some of Donncha's comments, it seems that if you have a ton of cached pages that can be inefficient as well.... but not really getting how or why.... plus it's difficult to see how many cached pages there are in total on a MU install since each sites dashboard will only list the cached pages for that particular site, it doesn't appear as though there's a way to see "total sitewide cached pages".

Any general rules of thumb or best practices regarding expire times anyone wants to share? Any thoughts on turning on "cache rebuild"?