The largest factor seems to be the temp where they are stored. If you can keep them refrigerated or very cool, like under 60 deg, MRE's are up to 7 years, I'd expect dehydrated even longer. I'm not sure about freeze dried, probably similar to dehydrated.

But if in heat, like your car trunk, MRE's 6 months., if that... and as you probably already know, some are inedible when brand new !! <img src="/forums/images/graemlins/shocked.gif" alt="" />