A dirty injector in any water softener can lead to a variety of problems.  Everything from just plain poor performance to excess water in the brine tank to not functioning at all! 
How often should you clean the injector?  It really depends on your water chemistry.  Water Softeners on municipal water supplies may never need to have their injector cleaned while well water with high amounts of iron and manganese where there is no iron pre filter before the water softener would need to have the injector cleaned every couple of years.  So really, whenever you notice that the water softener is giving you poor performance is time to clean the injector.
It should also be noted that not just water softeners need to have their injectors cleaned.  Iron and Sulphur filters, tannin filters etc also need this cleaning but fortunately all Clack WS1 valve filters use the same procedure.  This is a simple process you can do yourself, once you know how!
To clean the injector, first by-pass the water softener, then press and hold the regeneration button for 5 seconds until the timer starts to count down the cycle, this will release the pressure inside the valve so you can open the housing where the injector resides.  Open the housing and remove the injector, either using the cap as a pry tool or with needle nose pliers.  Soak the injector in iron out, CLR, vinegar or a similar product until clean.  You can clean out the orifice with a wooden tooth pick and a Q tip.  If you cannot get it clean you will need to replace the injector with a new one for the water softener to work properly.  Shop Clack Injectors here Once the injector is clean, press it firmly into place and close the housing.  Press regen again to advance through all cycles until you get to the end.  Then put softener back into service.