In this question about "Why don't trigonal S and P compounds undergo inversion at room temperature?", phosphines, sulfoniums and sulfoxides are described as "optically stable", while ammonia is not. From this question and elsewhere I infer that "optical stability" in this context refers to the racemization rate of the molecules being very slow. Is there a precise definition of "optical stability"?
Optical stability is a qualitative property rather than a quantitative one. If you want to describe the rate of racemization quantitatively, you can measure the Gibbs free energy of activation for the transition from one enantiomer to the other enantiomer. The higher the activation energy, the more optically stable the compound is.