Up to 8 days as per the kellymom link, depending on how cold your fridge is. Mine kept for that long when I was expressing. Best thing to do though is taste & smell it when it's fresh so you'll know when it's off
Just wanted to add a word of caution - some women find that it goes off faster than any of the guidelines suggest. It's something to do with enzyme levels, I think. The only way we can be sure my EBM is OK is to freeze it as soon as I've pumped it, and then defrost immediately before using it.
I'm afraid the only way to find out whether yours goes off fast or not is to taste it.