Hi,
I recently created such a system, please view its promotion site textsensor.com.
So how do we do that, you need to keep these points in mind
1) Cross domain ajax is not allowed so no sending data to your server when some user click on a ad that could be present in website of any of the publisher and you might got them in thousands.
2) Cookies might be blocked like you said, also different browsers might cause different sort of trouble :) when you deal with cookies in such scenarios .
3) You need to provide publisher with script tag that they will embed ads on there website for relevant keywords.
4) This tag must be executing server side programming language. for example
<script src="http://dennis.com/ads_application/get_my_ads.php">
OR if you have multple php files then include 1 js file that will import all php files into the website for example
<script type="text/javascript" src="http://www.dennis.com/ad_application/inline.js">
</script>
5) you need to tell server about publisher, so put there id and there campaign id inside script tag and put that script tag before inline.js file from stop 4.
<script type="text/javascript">
var ad_publisher = "5122";
var ad_campaign = "11129";
var ad_type = "inline";
</script>
6) you need to check whether current host is eligible for putting ads on there website
7) you can view sample inline.js file here we made for textsensor.com
8) For creating interval between when last user clicked on an ad, you need to record that user using there ipaddress and by creating cookies. that must be done on server side, that is send ipaddress to the server and time user clicked on the ad.