I do test a lot - mostly because Im still within the first year - so Im attempting to achieve some level of stability in order to keep my corals healthy. I suspect that as I move later into year 1 and on towards year 2 I can reduce the amount of testing.
Right now I test N03 PO4 at least 1x a day, sometimes 2x depending on whats going on.
Calcium 2x a week
Magenesium 3x times a week
Iodine 1x a week
OLI/Water clarity 3x a week
Alk: 3x a day
PH: Probe - tests multiple times a day
Salinity: Multiple times a day with probe - and usually 2-3x a week to check probe accuracy
Par testing: Usually about 1X a quarter
Temperature: I double check my temp probably 1x every 2 weeks to ensure that the probe is accurate
Other tests kits owned: I own copper, but dont use it that often. I also own most of the Hanna test kits which since the MasterTronic came along, I dont use as much anymore. In addition, I own several Alkalinity test kits to cross reference accuracy given issues I was having with the KH Director.
All that being said - the MasterTronic has been a great help in this regard to see the trends, and stay on top of the testing.