Frequency is a lagging indicator. Watch CPM.
Frequency tells you an audience has seen your ad too often. CPM tells you the auction already noticed. One of those is news; the other is a receipt.
The standard fatigue check is frequency. Watch it climb past 2, past 3, and somewhere in there you refresh the creative. It's a reasonable habit built on a flawed instrument: frequency is an average, and it's slow.
Why frequency arrives late
Frequency is impressions divided by reach over a window. To move meaningfully, it needs volume to accumulate — which means by the time a 7-day frequency ticks from 2.1 to 2.8, the fatigue that caused it started days ago. You're reading a summary of last week's problem.
Worse, it averages over the whole audience. A frequency of 2.4 can mean everyone saw the ad twice, or it can mean your core 20% saw it nine times and the rest saw it once. Those are different problems and the metric can't tell them apart.
Frequency is the audience telling you it's tired. CPM is the auction telling you it already priced that in.
What the CPM curve knows first
Meta's auction is continuously repricing your ad against how people respond to it. As engagement decays, your effective quality signal drops and you pay more for the same placement. That repricing shows up in CPM within hours — long before enough impressions pile up to move a frequency average.
The diagnostic pattern to look for is CPM rising while CTR falls. Read them together:
- CPM up, CTR down — creative fatigue. The audience has stopped responding and the auction is charging you for it. Refresh.
- CPM up, CTR flat — auction pressure, not fatigue. A competitor moved in, or it's Black Friday. Your creative is fine; your market got expensive.
- CPM flat, CTR down — audience mismatch, usually after a targeting or placement change. The creative isn't tired, it's in the wrong room.
The read that's hard to do by hand
Notice that none of the three patterns above can be read from a single metric. Each one is a relationship between two curves, judged against what's normal for that account — and "normal" is different for a €5k/month DTC brand and a €500k/month retailer.
That's why fatigue is one of the first things Adgent looks for each morning. It tracks the CPM–CTR relationship per ad set against your account's own baseline, and when the pattern turns, it says so — in words, with the reason attached, before the frequency column has noticed anything is wrong.
Frequency isn't useless. It's just confirmation. By the time it agrees with you, you've already paid for the lesson.