Actually, there are a lot of different variables for that answer. For one, it depends on the poison. Was it composed of natural things (plants and berries)? Does it have an alcoholic component? Are the ingredients chemical?
Generally, a poison doesn't have an expiration date. Powders may lose some potency, but are still useful. While liquid forms with alcoholic components do tend to become more toxic. Chemically manufactured poisons don't change unless something drastic in their environment does. Plant and berry poisons do have a tendency to become more deadly, but normally only in the liquid form. Let a liquid sit, it tends to stew. Let a powder sit, the effects aren't as extreme. If the container the poison is held in has an airtight seal, it should remain the same, but if it's put in the open the effects will vary.
there is my general knowledge in poisons, none of which I am making good guesses on.
should you be frightened that I have such an extensive list? Yes, you probably should. But whether I play off those fears is yet to be determined.