Ad frequency refers to the number of times an ad is served to an individual.
Research on ad frequency comparing traditional and online advertising shows that the more times an ad is served, the more likely it is for it to have an impact on people increasing brand metrics such as brand recall and purchase intent.
The questions are when does impact decline to yield diminishing returns, at what level should ad frequency be capped and over what time period. The ‘rule of thumb’ I have seen discussed most often is to cap at a frequency of 4.
This is display-based research, but the frequency cap suggested is much higher than most use and there is a risk of overloading and annoying consumers.