The index time is 'now'. There's no reason to use the past perfect, since you're not referring to more than one period of time in the past.
Now it's 11 pm and you were watching TV from 8 pm to 10 pm.
This is grammatical. You might use it in this context: "It's 11pm; you were watching TV from 8 'til 10, and you haven't even looked at your homework yet!"
If 'you' are still watching TV, you use the present perfect:
It's 11 pm and you have been watching TV since 8 pm.
(You could probably squeeze a grammatically correct meaning from your sentence if you added more context that required the past perfect.)
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