I've heard that storing AM liquid in the refrigerator extends the shelf life even more, but I don't know how much more.
Sadly, I just discovered the ClO2 pills I was using were out of date, about $70 worth of them. I tried to get AM to commit on storage life under ideal conditions (moderate temperatures and humidity, no package damage, etc.) but they wouldn't budge beyond the package date. The pills still fizzed when put in water. It broke my penny-pinching heart, but I threw them all out.