I have seen a number of topics on drupal.org discuss this problem without offering a real solution, so thought I would document my experiences in dealing with this for anyone who may have been struggling as I was.

On WikiJob.co.uk (my day job), we have the UK's largest and most active graduate forum. We've been trying to develop the job board side of our website recently whilst keeping the forums and community at the focus of what we are doing. Some recent feedback from some people we met at a careers fair suggested that some of our regular forum users did not even know we had a job board, so we decided to kill two birds with one stone. We have been making good use of the Drupal advanced forum suite of modules for a long while now but this is one thing we had not worked out how to handle.

In the advanced forum advf-forum-post.tpl.php you have access to a variable called $top_post. This is great, and sufficient if you want to do something like insert an advertising banner underneath the top forum post, all you have to do is something like the following:

if ($top_post) {
    ‹div class="my-advert-styling-class"›
	//advert link and image goes here
    ‹/div›
}

Nice and simple, and often sufficient for most people, especially if they are using Google Adsense as Adsense only allows three adverts on any one page. However we wanted something a bit more substantial. Our forum threads often run into the hundreds of replies on a single topic, so we wanted users to see a regular reminder or advert (in this case a hyperlinked image promoting our jobs board).

After much investigation and head scratching, I realised it is actually very simple. If you access $variables (the global variable of variables), there is a property called "id". This is initially set to one when you are on the original post, and then starts at one for the first reply and then goes to 2 for the second reply, 3 for the third reply and so on. So, if you wanted to output an advert after every 10 posts, all you'd have to do is something like this:

if ((!$top_post) && ((int)$variables['id'] % 10 == 0)) {
    ‹div class="my-advert-styling-class"›
	//advert link and image goes here
    ‹/div›
}

And there you have it, thats all you need for repeating Adverts on Drupal advanced forums :)