Translation:You were drinking milk in the kitchen two hours ago.
That means "You had been drinking milk in the kitchen for two hours" -- seit means something like "starting that long ago and continuing up to the reference time" (which is "now" when you'd use the present perfect continuous in English, and some past time when you'd use the past perfect continuous).
Was ist richtig?: "Du trankst Milch in der Küche vor zwei Stunde" oder "Du transkt Milch in der Küche zwei Stunde vor".
They're both wrong.
Try Du trankst vor zwei Stunden in der Küche Milch.
Expressions of time such as vor zwei Stunden "two hours ago" generally comes right after the verb, if they aren't the first thing in the sentence.
We don't put expressions of time at the end in German the way we do in English.