The performance life of a Transitions® lens will generally last the lifetime of the prescription which is normally 2 years. This is in keeping with industry standards for frames, coatings and other treatments that come on prescription glasses. After exposure to the sun for a period of time, it will eventually start to fatigue.
In most cases, Transitions lenses will darken even longer than 2 years, but it will depend on the amount of UV exposure to the lenses. Even after 2 years, the lenses are still suitable for indoor wear, if the prescription hasn't changed. Many customers like to tint them darker to use for occasional use sunwear when they buy their new pair of Transitions lenses.