I've never seen proof that 777 will simply degrade over time when sealed in a container and I seriously doubt that it does.
For me, proof would need to be in the form of chronograph readings that are verifiable from more than one source.
And the powder offered as proof would need to be from a container that was at least normally sealed from being exposed to humidity and the elements, and not powder that was left in a flask or loaded in a gun where it could be contaminated with lube or moisture.
It's possible that a 777 powder charge could last indefinitely after being loaded in a gun.
But any degradation could be contingent on other factors such as how much lube is in the patch, whether there's a card or unlubed wad separating a lubed patch or bullet from the powder, how clean and dry the gun is before loading, where the gun is stored, temperature fluctuations and whether moisture can get past the projectile or enter through the nipple flash hole.
If a person knows that their powder charge of any type would be left in the gun for a long time, they can ram some wax paper over the powder charge, or an unlubed wool wad or card.
And keep the nipple covered.
That's not a guarantee but it lessens the chances that any powder will be affected by humidity, condensation or lube migration from the patch or bullet.
Generally powder degradation is something that people don't worry about very much unless they're hunters or concerned about self-defense.
But maybe more folks should be aware that if they leave their guns loaded for a long time that they should take some precautions.
When folks talk about powder degradation, I think they mean that the powder can weaken a little bit, but not saying that they think that the powder will totally go dead to where it won't ignite at all.
Lube or moisture may be able to contaminate part of a powder charge and alter its strength, but that doesn't mean that the powder won't fire at all.
Although not igniting is always a possibility.
We don't really know if 777 is more or less susceptible to degradation or contamination compared to other powders.
BP is said to last forever, but the truth is that it can become contaminated very easily if left in contact with enough of some runny lubes to where it will result in a very weak load.
I would assume that 777 can too.
But we don't really know which powder will more easily become contaminated since every circumstance can be different.
My short answer would be that 777 doesn't go bad as long as it's sealed in the container or as a load in a gun that helps to keeps contamination away from it.
But the common practice is to shoot the load out of the gun at least every 3, 6 or 12 months based on a person's individual preference and previous experience with their gun.
The longer that any powder charge is left loaded in a gun just increases the chance that the load will need to be pulled if it goes bad.