Storage is where good arithmetic gets undone. A vial you calculated perfectly is worth nothing if it sat on a windowsill for an afternoon.
The general shape of it: lyophilized powder is stable and tolerant, and is usually kept refrigerated, away from light. Once you add bacteriostatic water you have a solution, and solutions have a much shorter useful life. Refrigerated, not frozen. Frozen and thawed repeatedly is how you damage peptide.
Bacteriostatic water contains benzyl alcohol, which is what allows repeat needle entries into the same vial. Plain sterile water does not, so a vial mixed with plain water is a single-use proposition regardless of how much is left in it.
Practical habits that matter more than any specific number of days:
- Write the mix date on the vial, not on a note somewhere else
- Swab the stopper every single time, including the first
- Add the water slowly down the vial wall rather than jetting it into the powder
- Swirl, never shake
- Keep the vial in the body of the fridge, not the door, where temperature swings most
The reason to record the mix date rather than trust memory is that it interacts with your dosing schedule. If a vial's usable window is shorter than the time your doses need to finish it, you should have mixed at a different concentration. Working that out before you add water is the point of a Peptide Tracker & Calculator, and the mix date belongs in the same record as the dose log.
None of this is exotic. It is just the difference between a vial you can trust in week five and one you are guessing about.