Keep in mind, your regular ROKU uses about 3GB for a 1-hour streaming a movie. The HD Roku uses 7GB for 1 hour.
In 10 hours a day streaming, that's 70GB. In 10 days that is 700GB. 10 more days and you are past the limit! Xfinity will charge you $10.00 for each 50GB above the limit (during the pandemic).
Yet Xfinity (comcast) says 95% of their customers use less than 1.2 TB of data per month. (During a pandemic? For real? And they are going to do this to us?)
If you have several ROKU devices in your house streaming movies at the same time, and kids on STEAM playing video games. Upload and download from cloud servers 80 GB at a time. Game backups. Kids doing school on the internet, Other computers and cellphones using WIFI. You doing work on the internet. ZOOM meetings. And people like me who watch a movie on the ROKU every night and fall asleep within 10 minutes while the DATA is burning away for 4 hours. You are quickly reaching your DATA limit way before the end of the month.
HOWEVER, (not confirmed yet) Xfinity has an "option" for you to decide on. It is this evil plan to jack up more people's rates and make it seem real nice. For a mere $11.00 extra pre month, you can have unlimited DATA and forget the $10.00 per 50GB over the limit. Is it worth it?
Most of Xfinity's service areas have had a 1TB cap for years, and they've had an option for unlimited for $30. (Used to be $50 until this year.) Recently, they've added a bundle where the unlimited is less if you bundle it with the rental of their gateway device. Now they are expanding it to the remaining areas (mostly the North East) and they've upped it by 200GB. (That's their pandemic acknowledgement.)
Is unlimited worth it? That depends… If you're going to go over a lot, it's certainly cheaper than paying overage charges. Then again, some people would say just cut the bandwidth usage and stay under.
By the way, Netflix says about 1GB/hour in SD, 3GB/hour in HD, and 7GB for 4K. These days a lot of TVs are 4K even if they are fairly small and this is probably a waste for many users so just limiting the streaming to HD will work for many.
We've been under the 1TB cap for years and never come near it though we only stream in HD and we don't have the TV on for anything like 10 hours in a day. (Except for the MST3K Turkeyday Marathon but that's only once a year.)
You might want to find a different way of falling asleep than 4 hours of video that no one is watching. Maybe use the radio instead, or OTA TV, or a Blu-Ray, DVD… Even streaming radio such as from the Tune In app, is typically around 0.05GB per hour, so if you fell asleep to that every night, it would add up to about 6GB per month.
Which is why I've never used a cable company for Internet access. I realize I'm fortunate to have another option, and many others don't. Hopefully when SpaceX has Starlink fully deployed the cable companies will see the writing on the wall and become more reasonable. Competition tends to make that happen.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.