This is a simple script using the Twilio PHP Helper library to retrieve Calls within a certain date range and the delete their associated recordings if they're particularly short.

targetted-delete.php

<?php
// Get the PHP helper library from https://twilio.com/docs/libraries/php
require_once '/path/to/vendor/autoload.php';
use Twilio\Rest\Client;

// Your Account Sid and Auth Token from twilio.com/console
$sid    = "ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
$token  = "your_auth_token";
$toNumber = "{fill this in with your number}";

$twilio = new Client($sid, $token);

$calls = $client->calls->read(
    array("to" => $toNumber, "startTimeBefore" => "2011-11-10")
);

foreach ($calls as $call) {
    if ($call->duration < 20) {
        echo "Call: " . $call->sid . " -- " . $call->start_time . "\n";

        $recordings = $call->recordings;
        foreach($recordings as $recording) {
            echo "Deleting: " . $recording->sid . "\n";
            $client->recordings($recording->sid)->delete();
        }
    }
}