Mozilla's documentation on differences between Safari's and Firefox's implementation covers the difference explained below albeit with a slightly incorrect description of Safari's behavior.

Safari: Granted storage access is aged out when website data is deleted by the tracking prevention feature itself which triggers after 30 days of Safari use without user interaction. Successful use of the Storage Access API resets that counter.

Firefox: Granted storage access is aged out after 30 calendar days regardless of usage patterns.

I don't think the age out is a crucial part of the API so I'm fine leaving it as a browser choice in the spec.