How long do tubes last in the box?
 

I found some old tubes still in their original boxes. They are over 15 years old. I know that tires degrade over time will the tubes still be viable?

Probably still good. I have a tube that still holds air in my MTB that has to be going on 15-20 years old at this point and I don't think being in a box would put any more strain on the tube, even where it's folded. Fill 'er up and see if it'll hold air!

It also depends on the temperature extremes they have gone through in 15 years

^ This. Whether installed or in a box, the conditions of storage are more important I'd say.

Should be able to sell as NOS for a premium!

Yes and no. Even kept in cool dry conditions, the paper used to make the box is a low grade acidic paper that will degrade over time. The acid in the paper degrades the cellulose which makes many different organic compounds that will react with NOx in the air to form ozone which will degrade the rubber. It would be difficult to predict the rate of degradation of the tube and high temperature will certainly cause the paper to degrade faster.

At this point, I'd say use the tube or keep it for another 40 or 50 years and try to pass it off as an antique. I'm sure there is someone somewhere who collects NOS bicycle tubes.

Yup! I knew someone would collect them.

Talking butyl: If concerned, I'd inspect it and then inflate it to ~2-3 times it's normal diameter (often done to find pinholes in leaky tubes). If it looks OK and withstands the test I'd have no qualms using them. I have some tubes that are ~10 years old and look perfect. I just use em.

In Michigan, they were kept mostly about freezing either in my bag or in our basement.
In FLorida, they were kept in a garage. The cardboard box is still intact with no signs of degrading. So I will blow them up and see. (I can't believe I didn't think to do that. D'oh.)