How to list commenters and days since last commented

It looks like you could add the

MAX( comment_date ) last_commented_date

to the field list to get the latest date, each user commented.

Then use e.g. the human_time_diff() function to get a human readable text for how long ago each user commented.

human_time_diff( strtotime( $result->last_commented_date) ) 

For safety reasons, you should then prepare the SQL statement with $wpdb->prepare(), since it contains the user input $amount.