According to some health studies drinking more than 3 glasses of milk everyday can harm your health and the study that was done shown that those who drank more than 3 glasses of milk per day were not only associated with mortality but also an increased risk of fracture and hip fracture.
Now that is just a study and I don't take those studies too serious as there's not really much way of knowing for sure if you're gonna die sooner or have a hip fracture etc from drinking more than a few glasses of milk per day.
We all are gonna die anyway and milk is healthy for you and sometimes I drink 4 glasses of milk per day if I want too and I'm okay and I'm not worried about it.
For what it's worth my Grandma lived to be 107 years old and she sometimes drank a gallon of milk in one day as she loved her milk.
Milk does help build strong bones as well so I would continue to drink as much milk as you want too and not worry about it.
Your stomach may begin to hurt if you drink too much milk well before you drank enough milk to do any actual harm.