I may have missed something in this thread, so please bear with me if I ask something that has been discussed already.
When you open a tin of tobacco, can it be resealed to prevent overdrying? I have a tin of MB Plum Cake that I opened months ago. I took out a portion of the tobacco and then put that and the closed tin in a Ziplock bag. Since then the loose tobacco has dried a lot.
It generally can't be resealed in the same tin.
The amount of time it will last at the proper humidity will vary from one tobacco to the next. Keeping it in the tin obviously helps it last longer than just being out or in a Ziplock. It does offer some protection, but will eventually dry out. I've done it. I've forgotten an open tin for a few months and came back to crispy dried out tobacco.
Aromatics with a heavy casing will last longer.
Thick flakes that come rather wet like Samuel Gawith or Peterson Irish Flake will also last a bit longer.
Ribbon cut tobaccos like Dunhill Elizabethan and the like will not last as long.
For me, if I'm going to take more than a couple of weeks to smoke it, I transfer it from the tin to a jar.
With my favorite blends that I know I'm going to smoke rather quickly I will just leave them in the tin.
I would say 2 weeks is a good time frame for a ribbon cut. If it's going to last longer than that, I jar it up.
The thick wet flakes could probably last a month without drying out to the point of not being enjoyable. I don't think I've ever put Peterson Irish Flake in a jar. I go through that stuff quick.