Does ad frequency actually predict creative fatigue?
Frequency (how many times, on average, a unique person has seen your ad) is a real signal that correlates with declining performance, since repeated exposure to the same creative tends to produce less engagement over time. But there's no single frequency number that signals fatigue in every account. The reliable read is a rising frequency trend paired with a falling click-through or conversion rate on creative you haven't changed.
Frequency alone, in isolation, isn't a reliable trigger. What varies from account to account is the threshold: a small, tightly defined audience can show fatigue at a lower frequency than a large, broad one, and video or carousel creative tends to hold up longer than a single static image. Campaign objective matters too, since a retargeting campaign is expected to run at a higher frequency than a cold-audience prospecting campaign.
The combined signal is what to act on: frequency climbing while click-through or conversion rate falls on unchanged creative. That combination points to two fixes, either refresh the creative or expand the audience so the same people are seeing the ad less often.