Based on this article, there aren't really clear guidelines on "leftovers" and there are a lot of factors that go into whether or not leftover warmed milk is safe to reuse and for how long.
http://kellymom.com/bf/pumpingmoms/milkstorage/reusing-expressedmilk/
There's always the smell test (if it smells like spoiled milk, throw it out). Personally, I only let it sit in room temp or in an air-conditioned room for an hour. Hope this helps!
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