Admit it -- this plugin reduces your blog's carbon footprint. :)
The plugin says,
Checking for and deleting expired files is expensive, but it's expensive leaving them there too. On a very busy site you should set the expiry time to 300 seconds. Experiment with different values and visit this page to see how many expired files remain at different times during the day. Aim to have less than 500 cached files if possible.
How about blogs that are rarely commented upon? How about the "not so busy" blogs with thousands of posts? Is having an expiry time of 43200 seconds (12 hours) inadvisable? If a half-hour expiry time produces a cache size of 100 pages, is it advisable to raise it five times? Or more? (Isn't disk space cheap?)
In short, what are the telltale signs that the expiry time is too short or too long?