A frequency cap is a campaign- or ad-set-level constraint on how many times a unique user can be served an impression of an ad within a defined window — per day, per week, or campaign-lifetime — enforced by the platform’s delivery system. It bounds impressions per user, not per session and not per click; “user” is whoever the platform’s identity graph resolves. The distribution it bounds is what creative fatigue tracks.
The cap is a budget-efficiency tool, not a brand-safety tool. Without one, the auction concentrates delivery on the responsive sliver of a small audience; the marginal impression there has near-zero lift but full marginal cost.
What operators see
A retargeting pool of roughly eight thousand users, running at a few hundred dollars a day on a conversion objective. Inside a week, the responsive few hundred users in that pool can comfortably see twenty to thirty impressions each, while the rest see almost none. The spend lands on the same faces, not on fresh ones.
Where a direct cap is exposed varies by platform, objective, and surface. Reach- and awareness-style objectives typically expose explicit controls; conversion-optimized objectives often do not, and frequency there is managed through audience size and budget. Labels and availability shift between releases.
Per-audience caps do not compose. Three retargeting ad sets — site visitors, add-to-cart abandoners, past purchasers — each capped at three per week will let a user in all three overlapping audiences receive roughly nine impressions. The cap is an ad-set lever, not an account guarantee.
The right number depends on objective: awareness tolerates higher frequency than direct response, where marginal lift on the ROAS curve falls off sooner.