Wait, you can only watch it once?
There are two options available to you. Rental, or purchase.
If you purchase it, at a higher price, then you can watch it over and over and over.
If you rent it, you have 30 days in which to start watching the film. After you start watching it, if you don't finish it within 48 hours, you lose it. Once you have watched it once all the way through, you have to buy the rental again.
The Steam rental is actually a stuff service. Most other digital rental services, like Amazon, Xbox marketplace, etc, give you 24-48 hours during which you can watch the film as many times as you like.
You know, as if you had gone to a Blockbusters and rented a physical copy.
