It really depends on the stability of one's system. When tanks are newly established or when coral are being added one needs to test more frequently to insure stable parameters and to insure that alk and ca are keeping up with coral growth. It also depends on livestock load and feeding patterns especially regarding phosphates and nitrates. I have a very stable home system (the zoo tank is another story!) and i have a good idea of consumption rates for my bioload. Thus i currently test ca, alk, mg, ph once a month or less as long as everything looks happy in the reef. I trust the look of corals (especially xenids) over test kits anyway.