Frequency ↑ with CPM ↑ → still bidding into the same narrow pool
Frequency flat with CTR ↓ → the cause is outside the creative
Where it matters most
Campaigns with small audiences, retargeting above all. A small pool pushes frequency up quickly, and without a cap you keep paying more to reach the same people.
There is no absolute line — the normal range shifts with category, campaign objective, and flight length. Comparing against your own account's past periods, and watching whether it is climbing faster than usual, is more practical.
Why does retargeting need frequency watched closely?
The audience pool is small, so the same people get hit repeatedly. Without a cap, conversion rate holds flat while CPM climbs and brand perception suffers.
How is frequency calculated?
Divide impressions by reach. 300,000 impressions delivered to 100,000 people is a frequency of 3 — the average person saw the ad three times. Impressions rising while reach holds flat means you are showing the same people more often, not reaching more people.