How long can we keep the unfinished breastmilk

Hello mummies! My baby is drinking 100ml nowadays but doesn’t always finish all the milk and sometimes left quite a lot. How long can we keep the leftover in room temperature? Can we put back in the warmer until the next feed? If let’s say i put 100ml in the warmer at 12am and she only wakes up about 1am to drink. Drinks about 30ml. Can i put back to the warmer and feed her on her next feeding? Sometimes she wakes up like 3-4am. Is it ok?