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50 years plus if properly made, sealed and kept at typical room temperatures, preferably air conditioned. Supposedly if it smells like vinegar if its started to decompose but I wouldn't swear by it. I suspect that powder makers and sellers have both financial and liability reasons for not claiming long shelf life, plus reloaders or resellers may mislabel and/or repackage old powder.
http://www.nap.edu/catalog.php?record_id=6289
Black and Smokeless Powders:
Technologies for Finding Bombs and the Bomb Makers
Authors:
Committee on Smokeless and Black Powder, National Research Council
Page 162 - "Stabilizer"
"A chemical incorporated in solid propellant to react with the decomposition products and prolong the shelf life of the propellant. Typically used at concentration of 0.5 percent to 2 percent. When properly stabilized, smokeless powder has a shelf life of nearly 100 years at 20 °C."
http://www.nap.edu/catalog.php?record_id=6289
Black and Smokeless Powders:
Technologies for Finding Bombs and the Bomb Makers
Authors:
Committee on Smokeless and Black Powder, National Research Council
Page 162 - "Stabilizer"
"A chemical incorporated in solid propellant to react with the decomposition products and prolong the shelf life of the propellant. Typically used at concentration of 0.5 percent to 2 percent. When properly stabilized, smokeless powder has a shelf life of nearly 100 years at 20 °C."