Shadow9mm,
The British military (the one for which I've seen the numbers published) limits the stockpile life of munitions loaded with double-base powder to 20 years and with single-base powder to 45 years. That will, of course, be worst-case conservative, and many of those powders would last longer, but it's an indication. Unfortunately, it is only a reliable indication for bulk-grade versions of a powder, such as military arsenals and large commercial manufacturers usually use. The canister-grade powders sold to reloaders must have a tighter burn rate specification than bulk powders. This is because the arsenals and large commercial loading operations (and even some small ones) have pressure test guns. They can test each bulk lot of powder they receive and adjust their loads to match the individual lot to their performance specification. But handloaders rely on recipes in books that don't magically change to match your powder lot, so the canister-grade powders for handloaders have an extra manufacturing step in which their burn rate is tested. Then they are mixed with slower or faster bulk lots of the same powder type to adjust the new lot's net burn rate down or up, as needed to achieve the tighter burn rate specification.
There are several strategies for acquiring those different bulk powder bun rates. Norma breaks each bulk lot into four batches and gives each batch a different amount of deterrent surface treatment. So they have four burning rates of the same powder every time and feel they can best arrive at matching performance in new lots by adjusting the ratios of those four. I don't know what they do with leftovers. Others use held-back previous bulk lots that were faster or slower than their target values and use those to adjust the new lot's burn rate. In this case, you don't know how old the oldest powder in your can is. It might start to break down sooner than you think if it is old enough.
If you look at the past lot recalls for different powder types, you will see these practices sometimes result in a very short life, which is unusual. Norma says they warrant powder for ten years if it is properly stored. That seems short if everything went well in the manufacturing process. Still, they don't have control over how warm it got in the shipping containers or at any other stage of the transfer from the factory to the handloader, so that is a bit of a CYA number. Because powder deterioration depends mainly on the rate of deterioration of the stabilizer in it, and that gets exponentially faster with the increase in storage temperature, I expect any powder stored in a freezer would likely outlast its owner and probably his offspring as well.