Deb,
Can't give you the whole answer, but I'm sure someone here will. I don't use natural casings, so this is only what I've read on the net.
They last quite a while, but I don't know how long. You rinse the salt off them before stuffing, and if you have some left over, I've always read you can re-salt them and they will be fine.
I've also read they always have a bad smell, but once rinsed in fresh water they are fine.
For now, I'm sticking to collagen casings--no fuss, just use what I need and throw the rest back in the box.
Hope someone gives the whole story soon.
drano