Materials stored in a liquid nitrogen dewar with a wide mouth are protected by the extremely low temperature of the liquid nitrogen or the gas that issues from the evaporating liquid nitrogen.
When all of the liquid nitrogen has evaporated, the temperature inside the unit will rise slowly to ambient.
The rate at which the liquid nitrogen will evaporate depends upon the pattern of container use and the age and condition of the container. Evaporation increases as insulation efficiency deteriorates with age and rough handling.
Opening and closing to insert and remove materials and moving the unit will also increase the evaporation rate.