Depending on the content you are watching and your TV resolution, it is possible, but does seem high. Unless ATT is able to do a device specific allocation, this 8gb could also be taking into account all the wireless activity occurring on your network (internet browsing, phones connected to wi-fi, etc)
Here is a pretty good article describing streaming data averages. https://www.androidguys.com/promoted-news/how-much-data-does-video-streaming-use/
Here is ATTs calculator https://www.att.com/support/data-calculator
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