In general, seeds keep much longer than most nuts in their original state. Once ground, however, seeds go rancid very quickly. Flax seeds are a good example: After grinding, they should be refrigerated immediately and used within 3 or 4 days.
Nature’s protective coating on hard-shelled seeds, such as flax seeds, prevents the body from absorbing nutrients. To get maximum nutrition, grind them into a powder with a coffee grinder or seed mill before eating.