Although most water conditioners do not list expiration dates on them. I would toss out stuff older than 5 years.
All chemicals break down and the purpose of your conditioner( a chemical) is to remove very bad chemicals from the water. Some people will not use stuff over 3 years old.
Personally I do not use anything over a year old if possible. I buy the size bottle that I will use in a year. If it last a little longer it isn't a big deal. I try and keep it as fresh as possible.
Do not use any conditioner that got hot from being in storage or other situation. Heat breaks down chemicals very fast.
If you leave a cap off your conditioner for a day or so it might not be as effective either.