When freezing milk in ice cube trays it takes around 4 hours for the milk in ice cube trays to freeze.
It takes around the same amount of time for milk to freeze in ice cube trays as it does for water to freeze in ice cube trays.
If you heat the milk up before freezing the milk in the ice cube trays then the milk will usually freeze within 2 hours of freezing it.
Cold milk takes longer to freeze in the ice cube trays than hot milk does.
Just like when you freeze ice with cold water it takes longer to freeze the cold water than it does for hot water.
If you want quicker frozen ice you should freeze the ice using hot water instead.