I've heard different things, most say around 7-14 days before it really starts degrading. But, there are things you can do to really slow that down. For one, you can reconstitute with the minimal amount of buffered acetid acid needed (genesispeptides.net) and then draw up what you won't immediately be using (within a week or 2) into insulin syringes and store in the freezer in single dose amounts. You want to avoid freeze/thaw cycles as repeating this process over and over will certainly destroy the peptide. Only freeze in amount you will use. Thaw out your intended dose in the fridge then draw up 2 parts bacteriostatic water to every part acetic acid just before use. This will lessen the "sting" from the acetic acid.
Some say using acetic acid will destroy the peptide as well and prefer to only use bacteriostatic water for the entire process. You might use this process to be on the safe side. Also, if you go the bw route, then only keep in the fridge what you will use within 5-7 days vs 2 weeks.