Why do reconstituted peptides last about 28 days?

That time limit comes from the bacteriostatic water. It has about 0.9 percent benzyl alcohol as a preservative. The preservative reliably holds back germs for roughly 28 days, which is why vials used more than once are usually dated to that point. After that, the risk of germs gets higher, even in the fridge.

The whole guide to mixing peptides, step by step, in the order problems come up

Educational use only. This database summarizes published research and is not medical advice. Where a dose appears it is a record of what a trial or clinic ran, or what people describe doing, with its source attached. Nothing here is a recommendation for personal use.

Part of shelf life after reconstitution. How long a peptide lasts once it is mixed, and how to store it.