I provided a lot of information on this topic in the above referenced CMP thread, which apparently, none of the subsequent posters visited.
Danger with Deteriorating Powder
http://forums.thecmp.org/showthread.php?t=157820
The bottom line is that gunpowder is deteriorating the day it leaves the factory. It is a high energy compound that is breaking down to a low energy compound. It is just following immutable thermodynamic laws. Temperature accelerates the breakdown of gunpowder, it follows an exponential function: which happens to be the Arrhenius equation. The hotter it is, the faster it breaks down.
Humidity effects gunpowder as there is a lot of water in the air. Right now, humidity is pretty high outside. Gaseous water condenses and evaporates on the powder grain, and the polar covalent bonds of water interact with the double bonded NO molecules. They also wick nitroglycerine to the surface which will spike the pressure curve. Nitroglycerine also attacks nitrocellulose, and it turns out double based powders deteriorate in less than half of the time of single based. Any ionic compound accelerates the deterioration of gunpowder. So, it ain't just water, but there is one lot of water around.
I don't know how dry to keep gunpowder or just how much humidity starts to effect gunpowder lifetime. Someone else needs to find data on this.
Based on the chart I posted on the CMP page, keep your powder below 86F and it should last 45 years for single based and 20 years for double based, assuming the powder was properly made. Powder lifetime is so variable that it can't be predicted with any real accuracy, so first world militaries pay people to go through their stockpiles and pull out munitions before they become dangerous to use, and dangerous to store.
http://www.smallarmssurvey.org/weap.../unplanned-explosions-at-munitions-sites.html This is a huge activity, about $1 billion in munitions are discarded by the US every year. I have seen estimates of hundreds of thousands of tons demilled per year, the US demill stockpile is 587,000 tons, and I think that is the back log.
http://www.smallarmssurvey.org/about-us/highlights/highlight-rn37.html Ironically, the anti gun, anti war group, the Small Arms Survey has the best open source information on munitions lifetime and explosions.
There are all sorts of old coots who claim their old gunpowder will last forever, because their expectations are they are going to live forever, therefore their powder hoard must last forever, but they don't actually know how much stabilizer is left in their powder. Some of these people are going to awake up to a burning house, assuming they wake up at all.
Old gunpowder will blow up your firearm and autocombust, burning your house down.