It wouldnt be useful to show only one ad to a specific visitor all the time. Instead, a certain ad should be shown only for a particular number of times, before being replaced with another one. Setting up that number is the process called frequency capping in online marketing. The main purpose of frequency capping is to prevent ad oversaturation, which would result in wasted impressions. If a user has no intention in the ad, theres no point of sticking to it any longer. Instead, a much better solution is to try advertising another product. How many attempts an ad will have depends on the preferences of the advertiser.