From the page that you posted from :
You have 30 days from the time of rental to watch your movie, and 24 hours (in the US) or 48 hours (elsewhere) after you've started viewing to finish it. Once the rental period expires, the movie will disappear from your iTunes library.
So 30 days maximum, but either 24 or 48 hours from when you start watching it.
May I ask a follow up?
If I download an HD movie to my iPad on a Tuesday, I will take up ~5 GB of iPad memory. I will play it Saturday. Once I am done playing it n Saturday, what do I need to do to get my ~5 GB of memory back? Does it happen automatically? Or do I need to manually delete the file? If so, how?