Cookie Window, what is it, what is the optimal length?

A lot of times we come across the term – “Cookie Window” or media folks talking – “There is a 30 day cookie window on this campaign”. What does that mean? This means that consumers who clicked on a digital banner or a search result have 30 days to return to your site and take the action that is being tracked. If the consumer had clicked on the ad, it will be shown in post-click activity and if the consumer had not clicked on the ad, it will be tracked as post-impression activity.
The optimal length of the window should be chosen based on each individual campaign or business, it should be tested and then the correct decision should be made. Typically, a click to conversion or action lag report is available from the ad server like DART or Atlas which shows the time tag between the click and the conversion or action. This is very important for DR (direct response) businesses which are trying to either sell something on their website or have a clearly defined action as then they can adjust the cookie window based on that.


As shown from the chart above, this is a clear increase in the number of actions up to day 10 and then there is a decline through day 15 and after that there are minimal actions. Thus, looking at this chart a cookie window of 15 is the optimal length for this business.