When this function is used, is the Interval unique to a particular Product Key or is it a global setting for all keys?
This is a client side setting. The sync is initiated by the client and not the server. It sets it for your app. If all the instances of your app set it to say 20 days than it will be 20 for all the keys.
Or, if you set it differently in your app based on some condition then it will be unique to a particular product key.