How long do your eyes need to adapt to darkness and reach full contrast?
Actually, the US Army and Air Force (and I assume the Navy) teach their life support troops about this (as well as the aviators), since so many missions are conducted at night.  It is actually some very interesting physiology involved in this.  This Army Instructional manual should be of help (PDF File).


  On an average it takes 30 to 45 minutes for your rods to be fully dark adapted to night vision.


It also covers things such as Mesopic vision and Scotopic vision, eye health, and how to actually dark adapt.  I suggest it as a good read.  It's less than 14 pages.