In a support document Apple notes that, along with the launch of the new Apple TV with 4K, it’s extended the rental window for films rented on iTunes.
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Customers have 30 days to start watching a movie after it’s been rented. Once a movie is started, there’s an additional 48 hours to finish the movie and/or rewatch it. That’s up from 24 hours previously (at least in the U.S.).
To see if a movie is available for rent, search the iTunes Store for that title and look for a Rent button. After you choose Rent, you can choose whether you want to stream your rental or download it.