I think the longer the tank has been set up, matures, becomes established and more stable, most folks probably test a little less often than they do when first starting out, or when setting up a new tank.
Also, the size of your tank vs. what you have in it may have an influence on how often you should consider testing.
If I make a change or addition to the tank, most any change at all, I test more.
First 4-6 weeks or so - pH, salinity, ammonia, nitrite and nitrate until I am sure cycle has finished. I've been known to test each day on a cycling tank ..... talk about feeling like you just wasted another test. But it has helped me to understand what's happening in there.
After that Calcium, Phosphates, Alkalinity once a week until I get everything where I want it, then about twice a month after that.
pH and salinity twice a week always - easiest and fast.
Nitrates 2-3 times a month as well.
After adding a refugium - another change to the tank - I am testing for nitrates quite often.
I always test when doing water changes.
It's a pain in the butt sometimes, but I find it helps me learn about mixing salts, freshwater sources and the outcomes.
Testing top off water is impractical - so I chose to get an RO/DI unit.