How long do inner tubes last?
Generally speaking, 


  Inner tubes will last forever, until you get a flat


Unless the tubes are very old - I'm talking 15+ years - you don't have to worry about them dry rotting, cracking, or loosing structural integrity. I have a collection of patched and new inner tubes from since I started riding around 9 years ago, and all of them look brand new.

And in my experience, the most common cause of flats is due to me doing a shoddy job of installing the tube (ie, the tube is pinched, stress on stem, etc). Back when I started working on bikes, I would regularly get flats a couple days after installing new tubes.

In short, if the tubes have lasted this long, and still hold air, don't touch them. I even remember reading (on Sheldon Brown's website?) that he has witnessed very old tubes bond to the tire.