iPhone push notification connection frequecy

Apple's documentation states

You should also retain connections with APNs across multiple notifications. (APNs may consider connections that are rapidly and repeatedly established and torn down as a denial-of-service attack.)

What does this mean in practice? How often can you open and close connections? Are you supposed to al开发者_如何学Pythonways maintain a connection between the APNs and server, even if you have nothing to send it?