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

[masaki]

... In a pouch, which is far from airtight, it will dry out like crazy after probably 3-6 months...

No, it will not dry out no matter how long the tobacco is left in the pouch as long as the relative humidity of the air is not 0%. The moisture content of the tobacco will decrease down to an equilibrium state with the surrounding air as time goes by. For example, wood's equilibrium moisture content(EMC) is 5.4% for the surrounding air's relative humidity 25%. I live in Japan and my room's average humidity in early Oct must be around 50%. I have no idea about the EMC of tobacco, but EMC of woold for 50% relative humidity is 9.5%. But, I felt the tobacco was more wet than 9.5%. I thought the tabacco has been soaked with some water absorbing agent such as glycerin.

Masaki

[TommyTree]

But, I felt the tobacco was more wet than 9.5%. I thought the tabacco has been soaked with some water absorbing agent such as glycerin.

Masaki

That would most likely be propylene glycol, which is found in many lower-quality aromatics. That could keep your tobacco moist for a long time, but many smokers choose to avoid tobacco that uses it, as it can goop up your pipes. It's probably safe to smoke, even after many years, but I don't know if time has any impact on how it tastes.