Unless otherwise stated, you have 30 hours from unlocking the film to start the film and a further 30 hours to finish watching the film.
If you have prebooked the film, you will have 30 hours from when it premieres to start the film and a further 30 hours to finish watching the film.
If are you unsure about how much time you have left to watch a film, you can find this info on the online film listing.