If I have already warmed up the breastmilk, and baby does not complete drinking it, how long can i keep it in the open for? Can I refrigerate after?

From what I understand, it is not recommended to reuse unfinished milk as it may be contaminate by bacteria after contact with your baby's mouth. Try putting a smaller amount into the bottle for each feeding If your baby consistently fails to finish the whole bottle. This will minimize wastage. If your baby want more, you can then prepare another bottle for her.