If you have access to a cron daemon, very. Create a cron job to run a delete script every 12 hours (or whatever ... depends mostly on your server load.) In the script, look for stuff that needs to be deleted (ie., age == int hours), then delete as appropriate.
Tougher, if not. Best solution I could think of so far for a similar situation W/O cron is to check-and-delete routine every time someone logs in. Or maybe every 10th page load for a given user ...
