A peptide that is stored incorrectly loses purity without you seeing any sign of it. Four factors determine how long the material stays what it was.
Temperature
Lyophilized material typically stays stable for 24 months at 2–6 °C, dark and dry. At room temperature that drops to weeks or a few months, depending on the sequence. Short transport times at ambient temperature are not a problem; that is exactly what lyophilization is for.
Light and moisture
Moisture is the biggest enemy of a lyophilizate. Always let a vial come to room temperature before you open it, otherwise condensation forms on the cold powder. Also store the vials in the dark; peptides containing tryptophan, tyrosine or methionine are sensitive to light.
After reconstitution
Once the peptide is in solution, shelf life counts in weeks instead of months. Store the dissolved vial refrigerated and protected from light. Bacteriostatic water contains benzyl alcohol, which keeps the solution usable for longer than sterile water.
Freeze-thaw cycles
Every round of freezing and thawing causes mechanical stress and concentration differences in the solution. So rather divide a dissolved vial into smaller portions than take it out of the freezer again and again.