Many manufacturers refer to the extinction time of their products, which is defined as the time required for the afterglow to diminish to one masb (0.032 mcd/m2, or about 100 times the limit of human perception). In practice, this is very difficult to see unless your eyes are fully dark-adapted and you are in a completely dark environment.
For practical purposes, luminances of one to two millicandela per square meter are more appropriate limits for life safety applications, and even this assumes a smoke-free environment. Thus, zinc sulphide products are useful for perhaps 30 to 45 minutes after their excitation source has been extinguished. The afterglow of strontium aluminate products, on the other hand, can be visible for several days or more.
At the other end of the time scale, strontium aluminate products can provide surprising amounts of initial afterglow. For example, a four-inch square of material held a few inches away from a magazine page can provide enough light to read by, at least for the first minute or so. In addition, microprismatic retroreflectors and other brightness enhancing techniques can increase the materials luminance by several times.