After you warm up breast milk the breast milk will last up to 4 hours before it goes bad.
When you warm up breast milk you should either feed the breast milk to your baby right away or place it in the refrigerator.
Never leave the warmed up breast milk out of the refrigerator longer than 30 minutes or so because the breast milk can go bad.
If you feed your baby the warmed up breast milk and there is any leftover you can refrigerate the leftover portion and then reheat it again to feed it to your baby.
It's recommended to not reuse breast milk that is leftover but I always kept the breast milk that was leftover in the refrigerator and fed it to my baby again as long as it was okay.
I never had any issues feeding my babies left over breast milk that has remained refrigerated or frozen.