As long as they stay sharp, I don't know why they would have an expiration date. It would be rare to have them rust in the package even if they're years old. I always run them over a sharpener once or twice just to ensure they're as sharp as possible once I open the package, but that's just how I like to do it.
Once out of the package and maybe they've been used once or twice, just sharpen and lightly lubricate with an oil if you want to, and you're good to go for however long.
Yep. Unless you've sharpened them to the point where you've removed a significant amount of weight, and they don't fly well, I don't think you'd ever need to get rid of them.