The short answer is: when it stops cleaning.
This all depends on your process though. If you are cleaning very dirty parts, your solution will not last very long as it can get saturated with contaminants which will affect the cleaning process. The solution can look dirtier than it really is which can lead some people to dump the solution before it is completely spent. A good test to see how dirty your solution actually is is to take a jug full of solution and hold it up to a light. When the solution is in the tank it can look a lot darker as there is no light getting down into the solution. By taking some out you can have a much better view on how contaminated your solution really is.
If you are cleaning especially fine items, you may want to change your solution more frequently to achieve a better, more consistent clean. This is down to each customer to decide though when establishing your cleaning process.
If you are looking to make your solution last longer, filtration on your tank may be an option worth looking at. Depending on the type of filtration you go for, they can be quite expensive additions to your tank. If you are cleaning dirty items in a large tank however, it can pay for itself within a year based on money saved from chemicals as your solution will last a lot longer and clean better.
To summarise, when you change your ultrasonic cleaning solution depends on each customer’s needs.
But if you are looking for a concrete answer: When the cleaning solution stops cleaning, it’s time for a clean solution.