Indoors and at night, photochromic ("Transitions") lenses are clear, while they darken in response to sunlight. Typically, during the first minute of exposure to sunlight, they will darken significantly, and then continue to darken slightly over the next 15 minutes. For the remainder of time spent in the sun, they will adjust as the light changes. Then, when you go into shade or indoors, they will begin to clear up, becoming significantly lighter in the first five minutes, and then gradually getting lighter until they're completely clear, usually between 10 and 20 minutes after going back inside. The lenses are meant to last as long as the prescription, which is between two and three years. After that, one may notice a residual tint regardless of whether there's sunlight; at this point, the lenses will need to be replaced. However, your lenses would need to be replaced either way at this point, as your prescription would be out of date.