Don't use a zip lock for storage, and unless the tin is a circular screw top I don't trust it to keep anything fresh and moist for more that a month. The oilyness of the leaf matters too, and I've had a tin of Odyssey open for a few months and it is still prime for smoking, even needs drying still....but that's another topic.

If I don't plan on finishing something within the month it goes in a jar. A natural blend (not soaked in PG) will seal itself in the jar due to the natural fermentation of the leaf. I have not boiled my tobacco jars as if they were jelly (and will never do such a thing, it's absurd and unnecessary).
Just stick whatever tobacco you have in jars and have peace of mind knowing nothing bad will happen to them.