The lifetime of small arms ammunition is basically determined by the lifetime of the gunpowder. If you used new gunpowder a rule of thumb is 20 years for double based, 45 years for single based.
Heat is the worst enemy of gunpowder.
If you are looking for more information, I found this UN manual.: International Ammunition Technical Guideline, IATG 07.20, Surveillance and in-service proof.
to be an excellent read with its explanation of gunpowder deterioration and the tests used to determine stabilizer content. Table one was excellent in showing how temperature reduces the lifetime of gunpowder “propellant”.
If it was not copyrighted, I would have copied and pasted the table.
Section 13.1 explains how gunpowder deteriorates, well worth understanding.