We used to be taught that 30 minutes would get most of your night vision back, but it takes 45 minutes to get it all back. Looking slightly away from the object also helps so that you're not focussing light on the Fovea which is the area of the retina you use when looking at something directly. The Fovea has a much higher proportion of colour receptive cells, and less of the more efficient "black and white vision" ones. So, if you need to spot something in darkness, look slightly away from it. The black and white "rod" cells are closer-packed, and more able to resolve movement, which is why you often catch sight of movement "in the corner of your eye".
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