Well sometime ago I ask the same question but in a different forum and this is what I got answer:
in fridge w/o bac water until the expiration date on the box and may be few more month after that date wish usually is like a year and with water and in the fridge around 2 weeks. And with water (mix) out the fridge probably few hours etc... I allways keep my in the fridge after mix but maybe some one here have a better answer