My vet has told me to only keep the bag a couple weeks after I have pierced it to use it. I leave mine pierced the whole time and just change the needles. Mine is like an IV, it has tubing that stays on the bag. I don't know if yours is like that or if you draw the fluid into a syringe.
If you are using yours with the IV tubing I would only keep it about 10 days like angieluv said. If you are using a syringe I am not sure.
I hope that makes sense. I wasn't sure by your post how you were using yours but thought I would throw in what my vet said for the way my bag is used.