Here is the official policy for you:

FLIGHTS WITH A BLOCKTIME OF 1 HOUR AND 30 MINUTES TO 2 HOURS AND 59 MINUTES WILL SHOW AUDIO/CoVision ONLY

For a while the cutoff was 2:30 block time but often movies were not finishing. Reason for this is because block time refers to "gate to gate". If you have 20-45 minutes of taxi, plus climb out and descent (20 minutes), you can see that there is barely time to finish the shortest length films (1:30) not to mention many films are 2:00 or more these days.

The decision was made based on negative feedback from pax who were paying for headsets but not seeing the end of the movie. FA's are subject to discipline for deviating from this, and while some may chose to, I don't think I'll take that risk.