How Long do you Keep Kalk Powder

[MitchM]

Mixed kalkwasser will absorb the CO2 in the air and make calcium carbonate which is useless to you.
The length of time it takes for this to happen will depend on how tightly sealed you keep your reactor.
I would add enough kalk powder to just keep a thin layer present on the bottom of the reactor and add more powder as needed only.
You shouldn't have to change the powder. It should all get used up.