Actually quite reasonable of a restriction. I've found stuff lasts about 2 weeks, but not everything expired at the same time, so that may not be a firm number (or I could have started looking at a time in the middle of an expiration period).
So if you are not trying to game the system, you can download at home, play on the bus (while connected via 3G) and 100% of playback will be from local, and nothing will ever expire. Also, if you open netlfix daily at home, but play offline at work, you shouldn't see a problem. So the "expiration" is over-stated in an attempt to generate clickbait to get people angry over the limitations that seem quite reasonable.