I believe that the shop manual is referring to the duration of the preglow light. At warmer ambients and warmer engine temperatures the preglow light will remain on for a shorter duration. When the light goes out, M/B believes that it is time for you to start the engine.

However, M/B has never tried to start an engine with 150K on the clock at zero degrees ambient.

So, the advice given by PEH is correct. The glow plugs will stay on for about 45 seconds based upon a timer. You may take advantage of this extended glow time, after the light goes out, if you wish to have easier starting in very cold ambients.