How long is milk good for?

Sarita • Pregnant with my 3rd. I have a 7 year old son and a 3 year old daughter 😊
If I warm up my breastmilk and only need half of what I warmed up, is it okay to put the rest back in the fridge for the next feeding as long as it wasn't used in the bottle used to feed him and just kept in the storage bottle? I know once it's introduced to babies saliva it gets bacteria.