Loaded Revolver
The powder will not degrade any faster in the cylinder than it does in the factory container. I am very meticulous in cleaning my revolvers as well as loading them. I keep 2 loaded in the house all the time. I usually fire, clean and reload them at least every 6 months or so. I have gone longer at times. Been doing this for over 30 years now, and I can honestly say I cannot remember a misfire on the initial charges at the range. I have had the typical problems (not often though) of a cap dropping in the action after firing. But a failure to fire due to powder - never.
Having said that, I will add my guns are pretty much kept in ideal conditions.
Exposure of the powder to dampness would be my only concern, more so prior to loading (that will be obvious). Your powder is not going to fail unless it is exposed to moisture.
I dug a can of powder out of a locker that I had forgotten about, been close to a year ago now. It had to date back to the early 90's. Still shot good.
I also shoot black powder cartridges. I have some of those that are several years old and they are as reliable as any smokeless cartridge.
The residue after firing is what is really corrosive. Never have had a problem with the unfired powder.
Short of fording creeks or going swimming with your gun, you most likely will be wanting to go to the range long before the load components go bad.
