Hi I am checking out the functioning of CPM networks. Quite clearly, CPM and interstitial networks rely on the to spend time on the page (and looking at the banner even if not clicking). So, is there a standard across the web for the minimum time a visitor has to stay on the page for the publisher to be paid? For interstitial ads, there are always a 'Close Ad' text as well. I am wondering if a lot of people clicking 'Close ad' in just a couple of seconds could be burning ad impressions without any returns to the advertiser.