To simply answer you question freeze dried food can last 25-30 years! In order to make you understand how freeze dried food can last so long its best to know the process of how food is freeze dried.

Freeze-Drying Process

PHASE 1: Freezing

The first phase, freezing, involves carefully filling large trays of fresh produce or prepared food and loading these trays into commercialfreeze-drying machines. These machines lower the temperature below water’s eutectic point, the lowest temperature at which the solid (ice) and liquid phase (water) can coexist. During the freezing phase temperatures are usually between -50 °C and -80 °C.

PHASE 2: Primary Drying

The second phase, primary drying, involves lowering the atmospheric pressure inside the freeze-drying machine through the use of a partial vacuum which speeds the sublimation process and applying just enough heat for the ice to sublimate which can be calculated using water’s latent heat of sublimation. For preserving food, the primary drying phase lasts several days in order to prevent the structure of the food from being altered which would occur if to much heat was applied.

Approximately 95% of the water content is removed from the food product in the primary drying phase.

PHASE 3: Secondary Drying

The third phase, secondary drying, involves raising the temperature from the primary drying phase and lowering the pressure further inside the freeze-drying machine in order to remove any unfrozen water molecules from the food product through the process of desorption where physico-chemical interactions between the water molecules and the frozen food are broken. When the secondary drying phase is complete, the partial vacuum inside the freeze-drying machine is usually broken by an inert gas such as nitrogen, to prevent spoiling, and then the product is sealed. In this final phase, up to an additional 4% of the water content is removed from the food product.