You can watch a rented movie anytime for a specific rental duration (usually 30 days after purchase but this can vary from case to case).
Once you start the playback for a movie in the rented bundle, it will expire after a specific duration (usually 48 hrs after playback but this can vary from case to case). You can find the details about the rental duration of a movie on the respective movie and bundle details page.
Note: Once you start the playback for a movie, you can watch the movie multiple times within 48 hours.