We brought a very small jar of vegemite back with us when we were in Australia in 1999. It is still almost full, smells and looks the same as it did the day we opened it. There is a Veggie Tales song that has "vegemite" in it, so we brought it back so our friends could see what it is. No one, so far, has taken more than a tiny smidgen of it.
Guess it'll last another 10 yrs or more. By the way, in Australia "biscuits" are cookies and "slice" is their term for what I would call bar cookies. The kind you bake in a pan like brownies and slice them to serve. (I noticed one of the posting used the term slice.)