Formally once a year, briefly every quarter. The two do different things, and you need both.
The annual review
The same questions as last year, answered on both sides, by several people, anonymously. Two hours of discussion, and a short list of agreements for the next year.
The point of "the same questions" is the series. One measurement is a snapshot and says little; three in a row show a direction, and that direction is the only genuinely informative part. See how to evaluate your agency.
Schedule that review before the contract renewal date, not after. Otherwise you are confirming an extension that had already happened.
The quarterly check
Twenty minutes, three questions: what went well, what ground, and what do we need from each other next quarter. No form, no score, but a short note afterwards.
This is the moment when small irritations come out while they are still small. Most relationships that break, break over something left unsaid for four quarters.
Why you should not wait until you are unhappy
A review that only arrives once you are already unhappy is not a review but an indictment. The agency hears a reproach rather than information, and the first reaction is defence rather than improvement.
It is also usually too late: what you raise has been going on for months, and the people who could have fixed it have since been moved to another account.
Two extra moments worth having
- After a large campaign or a launch. A short look back while everyone still remembers yields more than the same question six months later.
- When people change, on either side. A new account manager or a new marketing lead changes the relationship more than a new contract does.
The annual round is best run by someone from outside, if only because filling in an anonymous survey for your own manager does not feel anonymous. IKAg does that with COLLAB, under Agency evaluation & remuneration.
See also: how long an agency contract should run and what a normal satisfaction score is.