I've only been in this hobby for the past two years, so I don't have many jars that have been closed much longer than 18 months, but I'm sure others on this forum have tobacco in jars that has been there for a decade or more, and it's stayed fresh the entire time.
My one experience was that I recently opened a jar of FVF from 2014 that I'd bought from a forum member. It popped something fierce when I opened it, spraying tobacco dust everywhere, but also indicating that the jar had held its seal the entire time and the aging process had proceeded as expected.