If your granular garden food has been stored well and is kept well sealed in a cool and dry location then it can remain potent for 5-10 years and perhaps longer. If it has gotten damp and clumpy it will not last as long.  If it is an organic food that has soil microbes added, the nutrient level will be maintained assuming a well-sealed cool and dry storage but the microbes will have lost viability after a couple of years or so.  It is still worth using but that benefit will have been lost.