There's actually been reports on the 'net that, yes, it does eventually go bad. (Mine has yet to do so, personally, but then again, my copy of Silver lasted pretty much until last year, and even after it deleted its own save file, it still saves without a problem for one weird reason or another.) Crystal uses the same type of internal battery that Gold and Silver use, so it's logical that it might actually happen. It's just that it's difficult to say, as there's no specific set time for a battery to expire -- just an estimated time.