Riotblade

I've searched all over google for an idea on how this works. I'm trying to setup a page where people can only download a file every 20 seconds or so. The download is called from a function which checks if they have downloaded anything within the past 20 seconds. My idea is to get their ip address and insert it into a db with the date/time. What's puzzling me is how I can check and update the database every ten seconds. I was thinking of using a cron job but the lowest it can go to is minutes. I want to hear the opinions of skilled programmers on how to do this the most efficient way. Thanks