Well, when i first started my tank i tested every couple of days. Then i slacked off, but i started having algae blooms and Red Slime problems so i went back to testing, every few days or so, and now that the tank is nice and stable, i test maybe once or twice a month as long as i dont see an drastic changes. if i notice suddenly that i need to clean the glass more often than usual i'll test to make sure i dont have a problem with Nitrates or phosphates etc. I do my water changes religiously so i dont worry about things like pH, Alk, Mag, or Calcium levels. I use reef crystals to mix my own water at home and all the params of the freshly mixed water are usualy perfect. If you test too often it will start getting pretty expensive.
Hope that answer helps.