I began mixing DD's formula with cows milk at 11 months, whenever she had a bottle (about 3-4 a day by then) i mixed it 1 part milk to 3 parts formula for a week, then half each, then 3:1, then just cow. When she was 12 months i just gave her cups instead (by then she only had 2-3 cups of milk a day and NEVER drank more than half of any given cup, preferred water). So i guess i cold-turkeyed on the bottles but she'd been having sippy cups for some drinks for months and she never seemed to notice.

Bx