For movie rentals which are classified as either Library or New Release, you have 31 days to start to watch the movie from when you complete payment, and once you have started to watch you have 48 hours to finish watching.
For Express Release Movies you have 4 days to start to watch the movie from when you complete payment, and once you have started to watch you have 48 hours to watch.
You can watch as many times as you like within the 48 hour time period.