how do you separate creative fatigue from audience saturation?

when a facebook campaign starts losing efficiency, which signals help you decide whether the problem is creative fatigue or audience saturation? do you compare frequency, cpm, ctr, and conversion rate over a fixed window before refreshing creatives or expanding the audience?

If the CPA exceeds the target cost by 20%–30% for three consecutive days, I believe the creative assets should be replaced.

I prefer to evaluate trends over a few days instead of focusing on one metric. High frequency with a falling CTR usually means the creatives need refreshing. If CPM is climbing and conversions are dropping, the audience may be saturated. I normally test new creatives before touching the targeting.