"When do you drink milk?"
Translation:Quando bevi latte?
Oh my, it sounds awkward, but I cannot explain why! :D
If I would like to use you here, it would be to stress it, and I would write it at the end...
"Quando bevi latte tu?"
or (but the meaning would change "Da quando tu bevi latte?" (Since when do you drink milk, meaning "When did you start? I never noticed it)
In general, in Italian the subject can be always implied, we use it especially when we want to state that a thing is made exactly by that person, some kind of surprise or other feelings.