Any chemical mixture – compound or single chemical – has a definite span of time over which the chemical(s) become less potent.
The trick is knowing the rate of loss of potency (in other words, it takes 6 months for Belladonna to lose half its strength) and the normal strength of the poison. And that’s going to be different for a compounds, poisonous or not.
It’s a similar principle – although not the same – as radioactive half life.