Both my kids were off formula at about 10 months or younger, as you start to feed them solid foods they don't need it anymore because they can get everything they need from the foods.
Have you tried giving her juice?
Once my kids had juice they loved it!
Though my daughter refuses to drink just water so we mix some juice with the water.
Their doctor knows my son who's 11 months is off formula and has been for a while, never told me he had to be on it for a year or longer...
