If the lid is on you should be good for quite a few months. I recently found a bottle in the draw with the lid off and it smelt off. It must have had about 2 months open.
Was wonderign this myself, hada bottle of bubblegum juice in the cupboard for a good 2 weeks and i just cant taste the bubblegum anymore, more of a sherbet taste.
I've vaped stuff 2 years old and it's been fine but I do find that custards don't last as long as the tank crackers. Maybe it's something to do with the sweetener that's in em?