Typically, photochromic lenses darken substantially in response to UV light in less than one minute, and continue to darken a little more over the next fifteen minutes. The lenses begin to clear in the absence of UV light, and will be noticeably lighter within two minutes, mostly clear within five minutes, and fully back to their non-exposed state in about fifteen minutes. A report by the Institute of Ophthalmology at the University College London suggested that at their clearest photochromic lenses can absorb up to 20% of ambient light.
Because photochromic compounds fade back to their clear state by a thermal process, the higher the temperature, the less dark photochromic lenses will be.  This thermal effect is called "temperature dependency" and prevents these devices from achieving true sunglass darkness in very hot weather. Conversely, photochromic lenses will get very dark in cold weather conditions.  Once inside, away from the triggering UV light, the cold lenses take longer to regain their transparency than warm lenses.
Photochromic lenses filter 100% UVA as well as UVB. UVB light is more energetic and causes sunburn as well as skin damage including cancers, UVA light causes skincancers but not usually sunburn. UVB is blocked by all glass, UVA light is not blocked by ordinary windows or lenses glass.
A number of sunglass manufacturers and suppliers including INVU, BIkershades, Tifosi, Intercast, Oakley, Serengeti Eyewear, and Persol provide tinted lenses that use photochromism to go from a dark to a darker state. They are typically used for outdoor sunglasses rather than as general-purpose lenses.