Most people set OKRs quarterly. Some set an annual OKR as well, that acts as the large-scale goal for the year, towards which each quarter's progress is set. Occasionally, OKRs are set for just one month.
We'll discuss the benefits and drawbacks of the differing approaches below. For now, though, it's important to understand the relationship between your agency's overarching vision and the priorities you set yourself as you're going along.