The newer you are to the hobby, the newer your tank is and the smaller your tank is, the more often you will need to test.
New tanks: ammonia, nitrate and maybe phosphate.
Older tanks: Ca and alk regularly, Mg every couple of months... maybe nitrate or phosphate if I see any algae or if something dies and I can't find it in the tank.
Smaller tanks can get out of balance VERY quickly, so testing more frequently until you get very stable is the smart thing to do.
I have a 200g system that is just over 6 months old and I've been in the hobby about 12 years. Given the newer tank, but big system and all the experience, I test Ca, and alk every other week until I get stable results for a few months and then will switch to monthly testing.