I'd guess that after a few months or even weeks it's not as fresh as when the jar was first opened.
I would guess that it would stay fresher than most things once opened because of the ingredients.
And it probably contains preservatives which fresh mustard doesn't.
Well probably it doesn't contain any preservatives beside natural ones.
Mustard ingredients are: Distilled vinegar, water, #1 grade mustard seed, salt, turmeric, paprika, spice and garlic powder.