Checking for and deleting expired files is expensive, but it&#8217;s expensive leaving them there too. On a very busy site, you should set the expiry time to <em>600 seconds</em>. Experiment with different values and visit this page to see how many expired files remain at different times during the day. If you are using legacy caching, aim to have less than 500 cached files if possible. You can have many times more cached files when using mod_rewrite or PHP caching.