Here is how to change Akismet interval to three days instead of 15 days for deleting spam comments using the akismet_delete_comment_interval filter.

Normally in WordPress, Akismet deletes spam comments after 15 days, which may be fine with you or not. If you’d like to change the interval in which Akismet deletes spam comments, you can add the following code to your functions.php file:

 * Filter to change Akismet interval to delete spam comments after 3 days
 * Support/Donate:
add_filter( 'akismet_delete_comment_interval', 'custom_delete_spam_interval' );
function custom_delete_spam_interval() {
	return 3;

This changes Akismet’s interval to 3 days, after which comments held as spam are automatically deleted.