A stochastic formulation is developed for the time required for thr number of molecules taking part in a chemical reaction to change by a specified amount. With a reaction volume V, asymptotic expressions are obtained for the expectation value of this time and its corresponding standard deviation in the limit of large but finite V. It is shown that as V-->infinity these results become equivalent to those obtained by a deterministic approach, and a discussion is given of the regime where stochastic effects become significant.