Set frequency caps on standard ads
Set a frequency cap to prevent users from seeing an ad too often within a given time period. A frequency cap is the maximum number of times your ad can serve to any given user during the period you set in the Time period field.
Go to the Delivery properties section of your ad properties.
Enter a Frequency, which is the maximum number of times that the ad can be shown to each user during the time period you'll specify. The frequency can be any number from 1 to 15.
Enter a Time period: the number of minutes, hours, days, or weeks that your cap applies to. The maximum is 90 days (no matter what unit of time you select from the dropdown).
Frequency caps are available only for standard ads.
If you set a frequency cap for an ad, only browsers that have a DoubleClick cookie will receive that ad. If a user's browser does not have a DoubleClick cookie (or the user has opted out of DoubleClick cookies), then ads with frequency caps will never serve to that user. DoubleClick cookies help Campaign Manager avoid exceeding frequency caps. Without them, Campaign Manager can't be sure it hasn't reached the cap for a particular user, so it won't serve the ad.
How do frequency caps work?
Each time a user visits a page with your ad, Campaign Manager checks whether the cap has been reached in the time period leading up to the impression. The ad only serves if the cap has not been reached.
For example, if the time period is five minutes, Campaign Manager checks if the cap's been reached in the last 5 minutes (the five minutes leading right up to the present). If the frequency cap is 8 and the time period is 5 minutes, then the ad will only serve if there have been fewer than 8 impressions in the last five minutes.
Let's add a little more detail to this example. Say an ad request comes in at 10:09 am. Remember, the time period is 5 minutes and the frequency cap is 8 impressions. In this case:
Campaign Manager checks to see how many impressions have occurred from approximately 10:04 am to 10:09 am. That's the last five minutes. (It doesn't have to be exactly 10:09 am. If the impression happens at 10:09:23, then Campaign Manager looks back from 10:09:23 am to 10:04:23 am.)
If fewer than 8 impressions have occured, then Campaign Manager serves the ad, because the cap hasn't been reached. But if 8 impressions have occured, Campaign Manager waits until there haven't been 8 impressions in the last five minutes. The wait might take just one more minute. Or it could take several more minutes, if most of the impressions are recent. All that matters is that from the present to 5 minutes back, there've been fewer than 8 impressions.