Sodium chlorite solutions generally don't just quit working. Over a period of around 10 years then can loose some strength but this depends upon storage conditions.
The best thing to do is to run some tests to determine its current strength. Once you know that you can adjust the amount you use accordingly.
Sodium chlorite is a stable form of chlorine dioxide. By reducing the pH of it to a little under 2 you release all of the available chlorine dioxide as free chlorine dioxide. Using chlorine dioxide test strips you can measure the amount released and then figure out the strength of the solution.
If you have an accurate scale you can get a rough idea of the strength by accurately measuring 100 ml of your solution and weighing it. A 22.4% sodium chlorite solution has a specific gravity of about 1.2, so 100 ml of it should weigh about 120 grams.