Econometric modellers agree that the short term sales impact of a campaign diminishes with each exposure, although in some cases, there may be an s-shape at the beginning (e.g. for brands/products people are unfamiliar with).
This 'diminishing returns’ effect means that after a certain number of exposures, the ad will no longer be worth re-showing to a person if the aim is to generate an immediate sales uplift. It therefore makes sense to apply a 'frequency cap’ if the medium allows this. There is no way of knowing the optimal cap in advance but based on talking to people with experience, smartmarketing.me recommends capping frequencies at no more than 2 per week. The money saved can be use to extend the duration of the campaign or increase campaign reach by using an additional channel.
The exception to the cap is if someone has engaged with the ad indicating potential interest in purchase - in which case, the person can be re-targeted additional times to 'close the sale’ (e.g. up to twice a day).
For further insights about media planning, check out this short article or listen to this audio clip.