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

require 'Services/Twilio.php';
include 'credentials.php';

$toNumber = "{fill this in with your number}";

$client = new Services_Twilio($accountsid, $authtoken);

$calls = $client->account->calls->getIterator(0, 50, array("To" => $toNumber, "StartTime<" => "2011-11-10"));

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

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