How Long do you Keep Kalk Powder

[straightrazorguy]

Hi reefers. I run an Aquamedic kalkwasser stirrer to my 55 gal mixed reef tank (plumbed inline with a 35 gal refugium, on a reverse lighting schedule). I started it more than a month ago, and the water in the stirrer still looks nice and milky. I plan on introducing more SPS, and eventually have an SPS dominated tank. For now, I can keep the Ca at acceptable levels, but I have to keep dosing more and more every week.

Is there a limit to the kalk's efficiency? Does one have to change the powder in the stirrer? If so, how often?