Cows milk

Hello, just finished our last box of formula right before my little girls birthday. Wanted to start her on cows milk a couple of days early (I have been mixing it in with her normal formula bottles for a couple of days). How do you give it? Do I just give it cold straight from the fridge or shall I heat it up in my microwave in my mam bottles? Also how Iong can I have the milk out before I should chuck it away? for example, if baby is taking longer to drink the milk.