The shelf life of soy protein powder depends on several factors such as the manufacturing date, storage conditions, and the presence of preservatives. In general, unopened soy protein powder can last for about two years, while opened containers can last for around six months to a year. To determine if soy protein powder has gone bad, check for any signs of spoilage such as a sour smell, discoloration, or clumping. If the powder meets any of these criteria, it is best to dispose of it.