What is a recommended or allowable use-by date of pipe tobaccos?

[Mangyrat]

Thank you very much for the suggestions.
I challenged smoking the tobacco in question today. The physical characteristics of the tobacco seems almost same as before. I had no difficulties in stuffing them into the bowl and igniting. I enjoyed smoking it for a few minutes in my backyard garden until I found a tree infected by termites.
It has been six hours since I smoked the 20-year old tobacco, but I am OK as of yet.

So aged pipe tobacco attracts termites.