Use a cron job to run a PHP script that runs something like
<?
include "/thepath/connectionfile.inc";
$query = mysql_query("delete from table_name where .?.");
where .?. is some where clause to locate entries older than 14 or 30 hours. (Look up NOW() from www.mysql.com for example)
Then just run this cron job every hour or so.
If you have your own server you may as well run it frequently, but if your site is being hosted your hosters may not want you running frequent (even quick) crons, so it may have to be less often.
I run several crons on some quite large scripts once a day from "dubious" hosters and they don't complain.
