The process of oxidation is where the oils in the flax seed meal (because they are no longer protected by the hard shell of the original seed) react with the air causing it to go rancid and as a result is no longer any good for our bodies. How long it takes really is accurately said on the labeling of the packaging but you maximize the length by ensuring you keep you supply in a cool dark air tight place.
It is therefore not really advisable to buy flax seed meal in bulk because you are likely to struggle to eat your way through it all before it goes of; unless you have many mouths to feed of course.