Depends how stable your stuff is, what the load in the tank is and how good of a hobbyist that you are. I test about once a month for calcium and alk, unless the media in my calcium reactor is getting low, then every week, or so. I don't test for phosphate or nitrate except for when I might see some algae... but it always tests out at zero. I never test ammonia, nitrite or PH. Maybe once a quarter I will test magnesium.
I imagine that the answers you will get will vary a lot depending on the hobbyist and the type of test.