wp-super-cache v 0.9.9.3. Also wp-minify 0.9.2
I have set the preload refresh time to 4320- minutes (3 days) and expiry / garbage collection to 86400 seconds (1 day) but my preloaded cached files are over a week old. It doesn't bother me particularly since nothing has changed on my site since then but I thought you might be interested. Site is http://www.CADbloke.com.
Let me know if you need any more info.