The British military uses 20 years for double-base powders and 45 years for single base powder as maximum safe stockpile time. Actual deterioration can be far different. A few years ago there were reports of one lot of 4350 that was going bad in cans in just three years. Very odd, but several folks reported it. So just examine the powder.
The military used 18 months at 140° to intentionally use up stabilizers in one set of tests
. If you figure you roughly double the rate of deterioration every 18°F (10°C) increase in temperature, then at 68°F it should last 16 times longer or 24 years. At 57°F (wine cellar) it should last 37 years. That said, the single-base stuff often keeps working longer. And Alliant has some 100 year old Unique (a double-base powder) that is kept under water (it's original formulation wasn't deteriorated by moisture, but Alliant tells me that's not true of modern powder formulations) that still performs normally when a sample is dried and tested. So there are complexities imposed by other ingredients and storage condition factors.
The red rust color and acrid odor are pretty strong indicators of trouble, as is an oily surface that clumps the grains. I would not rely on lack of scent. The normal powder scent is due to evaporation of residual ethyl acetate solvent used to form the grains. If you store powder in a desiccated environment, that solvent will leave the powder faster than in a humid environment, so I don't think you can count on that smell as a consistent indicator of powder condition.