Go for a data allowance of 600 GB. Everyone in your household will be able to stream 1080p TV up to a cumulative of 10 hours daily, with data left over for all your other internet activities.
Pay attention to the video quality of the shows you stream—you could be using up to 7.2 GB of data per hour. More exactly, 720p uses 0.9 GB per hour, 1080p uses 1.5 GB an hour, and 4K Ultra HD uses 7.2 GB per hour.