I don't test much now 'cause my tank is getting older, but i used to test :
PH every 1-2 day(s), nitrates 3-5 day(s), ammonia when it smelled
, Ca weekly, Alk weekly, Po4 monthly;
It really depends on your livestock. If you have an overload of inverts, Ca, Mg, Alk should be tested often unless you have a Ca reactor