Its symptom is a falling click-through rate while frequency rises. The fix is usually not to raise the budget but to continue with a new creative.

In 14ads

A “creative is fatigued” alert is raised if frequency is 2.5 or higher and has risen compared with the previous period, and CTR has dropped by at least 25% over the same time. All three conditions are required together; if frequency is low or not rising, a drop in CTR is not counted as fatigue.

How the engine uses this metric from ad production to publishing is on the how it works page; credits and plans are on the pricing page.