Agree with jwh20 about an unopened bottle. As long as it's sealed, it should be good.
What I disagree with is to use what you are going to use out of the bottle when you open it and don't save what is left. Since it absorbs moisture as soon as it is opened. Since it does that, it just makes more sense not to worry about the open bottle and discard it appropriately. This may seem like a waste, but in the long run it is definitely just easiest. You just don't have to worry about how much water it has absorbed. If you size your new container size according to the job at hand, you actually won't waste much in the process. When I do this, it's usually less than 10% of the total.