Comment Expiration by Post Activity
Expire your comments when posts are inactive, not just after several days.
Post Expiration Settings
The default behavior of WordPress is to expire a post after 14 days. This feature changes that up.
You can now set up expiration based on post activity. If a post gets updated, or a new comment is posted, the clock resets.
Since expiration goes off of last activity, the number of days will be the amount without any activity.
You can choose to enable this feature for some post types or all post types.
By enabling per-post overrides, each post, page, or post type will have a sidebar option so that you can set the expiration at the post label.
Set Per-Post Activity Overrides