My guess is that his has been asked before but here goes.

I have some 25 year old Swiss. It has been stored in a cool (AC) dark, moisture free environment.
It is in the orgignal sealed plastic bottles.

Anyone has some data as to how long I can expect to be able to store this powder without degradation ... Thanks to all

Twenty five year old powder of any type, and especially black powder, is like brand new to me.
Black powder, being a mechanical combination of ingredients, literally, Never goes bad.
How long can you store it? It will be perfect for a
minimum of 100 years after you die. That should give you an idea of how long it is good.
If you aren't convinced, send it all to me for testing.

Thanks...

I didn't Want to give it all away but
I can include you in my will...!
No worries, I am OLD so you might see it sooner than you think...:-)

Ha, ok, you're on.
Seriously though, civil war powder is still good; and I personally have some duPont 4f in the 8 oz oval can, from the 1930s; it actually works better.

I still use DuPont Black 2f from an old steel keg and it works perfectly.