I started to offer a small amount of cow's milk to drink with her afternoon snack from about 10.5 months just so she could get used to the taste of it. Then from 11 months, I replaced 1 bottle a day with cow's milk, keeping the other 2 formula. Towards the end of 11 months, I replaced 2 bottles a day, with the last one still being formula. The day she turned a year, we just switched entirely to cow's milk. The recommendation is to wait until a year to make the switch, but I was confident she was eating plenty of healthy food, plus I was already offering a vitamin supplement since about 10 months when her formula intake dipped below 500 mls a day. Whenever you choose to start, I'd just do it gradually over several weeks or a month or more. Overall, it was an easy transition for us. She liked cow's milk so much more - no surprise, formula tastes terrible!