The answer varies with the age of a tank and what I'm doing with it.
For established tanks with a mature sandbed and a fairly constant bioload, I do little routine testing beyond salinity once I'm on a routine for what I am adding regularly to the tank -- food, makeup water, Calcium, and etc.
On the other hand, newer tanks for which the DSB critters are just growing out, and to which new livestock is being added, get daily checks of pH, and at least weekly monitoring of Alkalinity, Calcium, and Magnesium, plus as-required monitoring of Nitrogen (Ammonia/Nitrate/Nitrite).
I'll test more often than weekly in these newer tanks if I find parameters are varying from test-to-test. Once the results settle down and stay put for a few weeks you can safely start to relax and stretch out the period between testing.