The official answer is 28 or 30 days. That being said, I have used them much longer than that and not had any problems. Same with bac water. I would imagine that depending on the purity and quality of what you are getting, and whether there is any contamination, it will affect the usable longevity of your peptides.
Related rant: The tirzepatide I get from a compounding pharmacy adds some sort of preservative that allows it to last up to six months, but now ships it with an ‘extended best used by date’. So, basically, they are charging me full price for a peptide that had a shelf life of 6 months and has expired, and they just extended the expiration date by another 6 months bc their internal testing says it’s fine. But they aren’t sending me new vials that will be good for a year.