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.
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.