The essential Omega 3 fatty acid tends to break down very quickly, ground flax seeds bought commercially have probably lost all their omega. You still get the fiber benefits, but flax seeds offer so much more, its best to grind every batch fresh.

I like to mix my ground flax seed with unsweetened apple sauce and sprinkle with a bit of cinnamon. The pectin from the apple sauce is also good for the bowels. I also add some fresh ground raw pumpkin seed to the sauce and seeds to help irritate any parasites uncovered by the bowel cleanse.

Another way to use flax seeds is to soak some whole seeds in water overnight. The seeds will release a mucilage like substance that is an excellent intestinal lubricant and can make BMs much easier. Store the flax seed gel in the refrigerator and add 1 tablespoon once or twice a day to herb teas, water or juice, or yogurt to help with your cleanse. This works with any kind of bowel cleanse.

Edit-I almost forgot your original question. Your flax seeds will have a rancid taste if they get too old. Even whole seeds can become rancid, but not as quickly. When the seeds begin to taste "off" its time to replace them. Don't toss what you have, even without the Omega 3 oil, you still have a good source of fiber.