You can watch a rented movie anytime for a specific rental duration (usually 30 days after purchase but can vary from case to case basis).
Please note that once you start the playback, the rented movie will expire after a specific duration (usually 48 hrs after playback but can vary from case to case basis).
You can find the details about rental duration of a movie on its respective movie details page.
Note: You can watch the movie multiple times after you start the playback within 48 hours.