This totally depends on the stirrer size and how much you are dosing. In a big stirrer where you dose only a small amount of kalk it could last a long time, but in a small stirrer with lots of kalk going through it you will need to add kalk more often. For me I add a little over 2L of kalk a day and I need to add to the stirrer about twice a month. The stirrer is DIY but measures 4in in diameter by 2ft tall.Ok. Watched a few videos and think I have it figured out. My last question is how much kalk do should you have in the stirrer and how long does it usually last?