The official expiration date on the bottle is about 2 years. The N will last much longer than this if properly stored. If you don't open the bottle and store it in a cool dry place, the N should last for many years . If stored properly, the N only degrades about 1/2 percent a year. When the N goes bad, it will turn a milky color, and you will see little particles floating in it. Unopened bottles of N have been tested after 20 years and have been fine. Personally if I waited 20 years to open my bottle of N, I would send a sample to a lab and have it tested before I used it just to be safe.