It uses the origin’s caching headers, such as Max-Age, as hints in deciding whether a particular document or resource is stale. When a user makes a request for something that is stale, that request causes a new copy to be fetched, so that the next user gets fresh content.
To limit the amount of load it generates for publisher sites, the Google AMP Cache considers any document fresh for at least 15 seconds, and any resource fresh for at least 1 minute. Note that those numbers may change in the future, as we tune the cache for optimum balance between freshness and load on publisher sites.