wpDiscuz comment plugin settings are located in Dashboard > Comments > Settings admin page. The seventh tab is the “Cache” tab. It includes all settings related to comment author avatar caching.
Enable Grvatar caching
When this option is enabled, wpDiscuz creates copy of avatar images in local file system and next time it loads images from local server. This excludes the extra loading time from third party servers like Gravatar and makes comment list loading faster.
This option allows to chose the caching method. The runtime method caches avatars on page loading process. The cron-job method based on WordPress cron-job functions and caches periodically based on WordPress cron-job time settings.
Cache avatars for “X” days
Using this option you can set the cache resetting period. This deletes cached avatars which are old than X days.
Cache purging buttons
Besides the automatic avatar cache deleting functions you can purge expired cache or all caches using the according buttons on this setting page.