Ad fatigue is an audience seeing the same creative until they stop noticing it. A real force in social and display, where you interrupt the same people repeatedly. Search mostly escapes it, because each search brings fresh intent: nobody's 'tired' of your ad for a problem they just got.
What decays in search instead: message drift (your ad slowly mismatching how people now phrase the search) and competitive staleness (rivals copying what worked). The refresh cadence is quarterly review, not weekly creative churn.
If you run display or social remarketing, fatigue is real there. Cap frequency and rotate creative. In search, spend that energy on the search-terms ritual instead.