30 days, or one time to watch it.
If you rent movies from iTunes, 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.
But if you start it, you only have 24 hours to watch it completely. Then its gone.