Humidity is the enemy of Blackpowder so, if the gun is stored in a cool, dry location the powder should be ok for a long time. I'm sure you've heard the expression; "keep your powder dry". That's why.
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The NRA tested a .31 pocket revolver that had been loaded since the Civil war and found the powder still good. They pulled and replaced the bullets , replaced the caps and fired all five chambers. This was in the Rifleman a few years back.
It sounds like you are considering using a blackpowder cap-and-ball revolver for home defense. A better alternative might be to buy a "conversion cylinder" for it that will allow you to use .45 Colt ammo. They are not too expensive and don't require any FFL paperwork. You are limited to fairly low pressure ammo in them but a 250 gr bullet at even 750 fps is pretty effective.