Apple Music allows you to download an unlimited number of songs onto your phone or computer and then listen to them while not connected to the internet. What prevents someone from subscribing to Apple Music, downloading a large number of songs, and then stopping their subscription?
I assume that when you try to play a song, even one that is downloaded, the software (iTunes or the iOS Music app) checks your account status to ensure you are still an Apple Music subscriber. But what if you keep the device off of the internet? Apple would have no way of knowing if you are still a subscriber or not.
The only thing I can think of is that the songs have some sort of timestamp on them tracking when the service last verified your account; and that they will stop working after some period of time if it is not verified again. If this is how it is done, what is that time period? How long can you go without connecting to the internet while listening to a song downloaded from Apple Music?