Welcome to FlyerTalk!
As you spend more time here, you'll see that FT is divided into forums for each airline and a variety of other (mostly travel-related) topics. With a quarter million members and over ten million posts, that's the only way to avoid total chaos.
Airline policies regarding mileage expiration vary. There is no standardization.
Your question applies only to US Air. There's a forum for it. There's a good chance that it has already been discussed there. You could try a search using a word like "expiration" that's unlikely to appear much in any other context.
If it hasn't been, that's where the Dividend Miles experts hang out (though one of them just posted here). I'm going to ask a moderator to move it over.