If you stream a lot of Netflix movies and shows then you can use a lot of data.
Streaming an hour long movie on Netflix uses on average of 1 GB of data but could be a bit more depending on the quality of the video.
If the Netflix movie is a good quality movie that you're streaming to your TV then you may use a bit over 1 GB of data.
Watching a 30 minute show on Netflix may only use a half GB of data or less depending on the quality.