I think after 2 - 3 hours of no activity on screen the app then throw's a message on the screen indicating if you are still watching. If you do not hit the 'Yes I'm still watching' message, the app shuts down. From what I can see in the settings app, there are no options for this setting.
I haven't seen a way to disable the warning message either...same as Netflix and other streaming services. They don't want to use their servers to stream to empty rooms so they make you perform some action (click yes I'm still watching) to continue the stream.