depends on many corals u have, If you have a lot then i would test it every couple days, or at least until you are confident you have some type of Alk additive dialed in. If you have dont have a lot of corals but have a few then i would test it weekly until you get a feel for the actual consumption. Frequent testing is going to lead you to learning your alk consumption to the point where you can pretty much guess whats it going to be after awhile. I test everything before a water change and i test the water that i have just made for the water change.