How long is a reconstituted vial actually good for?

The storage post explains the mechanisms clearly but does not give a number of days, which I assume is deliberate. Is there a way to reason about it rather than looking for a number?

Deliberate, yes, and the reason is that a number would be fiction. Stability is per-molecule, per-formulation, per-temperature, and per-how-many-times-you-opened-it. A single figure covering all of that would be wrong for almost every case it was applied to.

What you can reason about:

Sequence-dependent. Peptides with methionine, cysteine or tryptophan have oxidation-prone residues. Certain adjacent pairs are hydrolysis-prone. So two compounds in identical conditions can have very different real-world shelf lives, and the difference is in the sequence.

Concentration matters. More dilute is generally less stable — less material to lose before the loss is proportionally significant, and more surface interaction per molecule.

Every access costs. Temperature excursion, an interface, a puncture.

Bacteriostatic buys microbiological time, not chemical time. It does nothing at all about hydrolysis.

The practical version: a fortnight cold and undisturbed is a different proposition from two months of daily access, and neither has a number attached.

So the honest answer is a set of variables rather than a duration, and anyone quoting a flat number for all peptides is not distinguishing between them.

Which is why the "28 days" figure that circulates is worth being suspicious of. It is borrowed from multi-dose vial microbiological convention and has nothing to say about whether your particular molecule has chemically survived.

Correct, and that is a good example of a number that is real, is about something, and is about a different thing than the one it gets used for.