It can depend on the film, it's popularity, timing as to coming out on DVD to public. Usually, from my experience, I would suggest from their prime box office run 3 to 6 months, up to a year for the biggest grossing films. Some airlines will choose to offer films made in the home country or language or connect better with the home country of a given airline.
There may also be some time needed to edit a film for airline use not just as to all audiences, but also specific airlines, systems used, countries served by a flight or airline. Some countries have restrictions, censorship laws or require modifications of films as to violence, sex, references to abortion, alcohol, illegal drugs, references to certain ethnic and religious groups, as well as some humorous references. Some changes may be done so that most people can understand a film. Now if one could avoid seeing box office bombs from being shown when one doesn't have a choice on a flight