| I would think of the fish food a lot like cereals. Both are dried products so they have a pretty long shelf life compared to fresh foods. The issue is not with spoiling and being harmful, but rather spoiling and being gross. Because the fishfood has fats, proteins, and carbohydrates it is vunerable to several different types of spoilage. That being said, I would give them 3-6 months.
The best way to buy fishfood Ive read is to buy smaller containers that you would use up rather quickly as opposed to in bulk. It also reduces the risk of a potential harmful type of spoilage from developing, although I dont think that is a huge issue.
*disclaimer: Anything containing proteins, fats, or carbohydrates is at risk of developing into a bacterial risk. Bacteria use all the components of food I listed and can rapidly grow under the right conditions. Molds can also grow on these nutrients and produce a large array of toxins. Keep all fishfood that is dried, dry. Moisture is all the microbiological intruders need to grow on any dried food product. | 
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