From the FAA website:
Glass photochromic lenses (PhotoGray® and PhotoBrown®) automatically darken when exposed to UV and become lighter in dim light. Most of the darkening takes place in the first 60 seconds, while lightening may take several minutes. Although most photochromic glass lenses can get as dark as regular sunglasses (approximately 20% light transmittance in direct sunlight), their darkened state may be lighter due to the reduced UV exposure through the windscreen. In addition, the faded state of photochromic glass lenses may not be clear enough to be useful when flying in cloud cover or at night.
http://www.faa.gov/library/reports/m...a/F2003_03.pdf