You can safely use it up to 4h after it was initially warmed, and you can rewarm it once.
"... If the milk is warmed up but not used, it is OK to reheat the milk once. But the more you reheat the more you decrease some of the valuable immunologic properties of the milk..."
(
http://www.kellymom.com/bf/pumping/reusing-expressedmilk.html)
Breastmilk is naturally antimicrobial, which is why you can reuse it (unlike formula, which would have to be discarded). Just be careful if you do top it up with fresh bm for the next feed - at that point it would have to be discarded.
For example,
7am feed 6oz bottle, only drinks 4oz
10am feed remaining 2oz mixed with 4oz fresh, drinks 5oz
remaining 1oz would have to be discarded since it contains milk that is older than 4h
Not sure if I explained that well?