The transition speed depends on the specific lens technology and temperature—generally, photochromic lenses begin darkening within seconds of UV exposure, reach full tint in under a minute, and return to clear within two to three minutes indoors, with premium versions offering faster fade-back to avoid the "indoor sunglasses" effect. Modern photochromic lenses are also designed to maintain performance for the lifespan of the prescription, typically two to three years, as high-quality lenses use advanced photochromic molecules embedded in the lens substrate rather than coated on the surface—ensuring consistent darkening and fading without significant degradation throughout their intended use.