Is it latex or butyl rubber? How many tubes are we talking about? Do you patch your tubes? If it's butyl I wouldn't worry. I've had a dozen tubes lying in my garage (and sometimes with me riding) for +four years w/o any problems. Avoid storage in a sunfilled place, though. Put them in a dark box, instead :-D Latex is lighter but a lot more fragile and short lived (both mounted in the tyres and on the shelves).