Indefinitely, if you never crimp them.
They don't expand so they never have to be resized. There are only a few documented smokeless loads out there, mostly with red dot. They are on the internet if you search. Alcan had a load brochure with some loads from the 1960's too- I've got it, but I don't know where it is now.
Barney Fife: "Nip it, nip it, nip it!"
Andy Griffith:"Oh now Barn'...."