GH in powder form NOT MIXED will stay over a year in a refrigerator. If you ever looked an exp. date on a legit box and its fairly new, exp. date is about 18 months out. So like Finny said most likely 2 years stamp on it at the manufacture plant. If you have generic with know exp. date I would say its most likely good for at least a year or would try to use withen a year.
Once mixed and kept in the refrigerator it should be used up within a week, although if you get a US made GH like Genotropin it says on their inserts the preservative they use allows the GH to last 28 days while refrigerated. I don't think it regular BAC water though.