Definitely agree with what xandros9 has said up. It all depends on the quality of the movie. If you watch HQ or HD movies it will hog up your data like a hungry lion. Try watching a little bit of a movie and you will get an estimate of how much data it uses and then you can work out your figures.
I have seen my friends watching High quality Youtube videos and then screaming their data plan is exhausted. Anyone would recommend using high data usage apps, games, movies etc on a Wi-Fi.