Ad fatigue is the drop in response — falling CTR and rising CPMs — that sets in when the same people see a creative too often. It is diagnosed through frequency: the average number of times each person has seen the ad (impressions ÷ reach).
Why it matters for agencies
Every audience has a frequency ceiling beyond which extra impressions stop adding response and start eroding it. Watching frequency tells you when to refresh creative or widen the audience rather than spending into diminishing returns — the difference between a campaign that scales and one that quietly burns budget.
Run the numbers with the free ad fatigue / frequency calculator.