A few issues:

[eluser]bastones[/eluser]
You could do it based on "up to x amount of clicks per month", and for every click you add +1 to the database. I guess there's no even way of doing that but in the meanwhile you can use the rand() PHP function to randomise which ads are being displayed such as:

Code:

<?php
$i=rand(1,5); // 1,3,1,2,5 etc
echo $i;
?>