The amount of data that an hour of Netflix uses is about 1 GB per hour but depends on the video quality that you're streaming.
Watching Netflix movies and shows can easily cause you to blow through a limited data plan so if you're on a limited data plan make sure to watch your data usage closely.
You may also need to lower the video quality to save on your data.
A 1 hour high quality movie on Netflix uses up at least 1 GB of data but can sometimes use a bit over 1 GB of data.
Netflix is a good way to watch movies you want when you want and it's best to have an unlimited data plan when watching Netflix but if you don't watch many movies on Netflix then even a 50 GB or 100 GB data plan may be enough.