I did it in March for a month and got good health benefits from it. My cycles improved, my thyroid improved (but needs more work), my sleep improved, my eyes are less sensitive to sunlight, my mental clarity improved. I drank about 1 1/2 gallons a day of Organic Pastures milk.
My dad has been on what I call "The Cheater's Milk Diet" which means he's all milk except when a friend invites him to lunch. He's actually lost about 20 pounds. For weight loss, McFadden (below) recommends drinking about 3/4 gallon a day. I've tried it and just didn't stick with it. I've been too busy lately to focus on a "diet."
If you are going to do it, get a hold of one of the milk diet books. OP sells one by Charles Sanford Porter and you can often find one by Bernarr MacFadden on Amazon. I have actually been planning on setting up a milk diet website and offering both books there. And like many other things, I haven't done this yet.