It depends which stage the vial is in
A refrigerated, unopened, lyophilised peptide vial can typically last months when properly stored and protected from light. A refrigerated, reconstituted vial is a different story: most guidance points to a window measured in weeks, commonly cited around 2-4 weeks for many peptides.
Why the gap is so large
Reconstitution introduces water, which is the medium most of the chemical reactions that degrade a peptide actually need. Removing that water, as lyophilisation does, is what gives the dry powder form such a longer window in the first place.
Factors that shorten the window further
Temperature consistency matters as much as the average temperature. A fridge that regularly swings above 8°C, or a vial repeatedly moved in and out of the fridge, degrades faster than one kept at a stable, cool temperature throughout.
Light exposure
Most peptides are sensitive to light as well as heat, which is part of why vials are typically packaged in amber glass or opaque containers. Storing a vial somewhere dark, even within the fridge, adds a small extra margin.
The practical answer
There’s no single number that applies to every peptide and every storage situation. Checking the specific product’s own documentation, and defaulting to the shorter end of any published range when in doubt, is the safest approach.