It really depends on what's going on in your tank.
I've done 1 water test in the last couple of years and that was last week for ammonia because I pulled some plants out of the substrate which disturbed things a lot, and threw a considerable amount of detritus into the water. I did 3 x 50% water changes back to back to remove as much as I could, and the following day I wanted to see if I needed to do more or if I had effected my cycle in any way.
If you are cycling a tank you should be testing daily. If it's a newly established aquarium you might want to test prior to a water change to ensure your parameters are stable. Once you are comfortable your aquarium is established and your parameters are stable put your test kit away and get it out again if something happens to make you suspect something is amiss. Observe your fish, they are a better guide of things being off than a test kit, and use a test kit to confirm what that might be.
If you are running a fish room with dozens of aquariums you might want to continue with frequent testing because it's more difficult to keep track of what's going on in multiple tanks than it is with smaller numbers that you can give more individual attention to. If you are adjusting water parameters from what comes out of the tap to suit your fish, you should always confirm through testing that your parameters are consistent after your water change. Other than that put the test kit away. They aren't all that accurate anyway. It's a home test kit, not laboratory testing. Some of the tests aren't even for what they say they are on the bottle. Most ammonia tests are tests for Total Ammonia Nitrogen (TAN = free ammonia + ammonium) when all you are really interested in is free ammonia. APIs carbonate hardness test is actually a test for alkalinity (they are similar parameters, but not the same). Tests for nitrate are notoriously innaccurate. Even laboratory testing is difficult to get accurate because so many other factors can throw off a nitrate test.
I'll give you an example of innaccurate testing. That ammonia test I did last week showed some ammonia. It appeared to be in the range of 0.25 to 0.5ppm. Can anyone honestly say they can reliably distinguish test results apart? Move into a different room or go outside and they will look different. There are specific conditions you need to view these tests, against a white background in natural light. It's not uncommon for the ammonia test to show slight ammonia on a known ammonia free sample. This could be down to general inaccuracy, not doing the test in a 100% manner, other contaminants, or in my case the test kit hadn't been used for a couple of years, so these test kit go off over time and expire. I'm not buying a new kit for what might be 1 test. So I get some bottled water that will be zero ammonia and test that. If the bottled water test looks the same as my aquarium water test I can call the aquarium ammonia zero. Which it did. But even if there was say 0.5ppm ammonia (TAN), at my temperature and pH, that's only 0.00655ppm free ammonia. As a general rule free ammonia starts to get toxic at 0.025ppm. In my aquarium (temp 24c/ pH 7.4) the result of my ammonia test (TAN) would need to be 2ppm before the free ammonia bit gets to 0.025ppm so TAN of 0.5ppm isn't really an issue beyond saying something had effected the cycle and you should continue to monitor things.
The idea that any ammonia in your test is harmful is just false, but its a useful simplification for someone who doesn't understand water chemistry to ensure they aren't harming their fish. But if you don't really understand water chemistry, and can't really rely on the results, too much testing isn't always useful. Just use it as a guide as to where things might be wrong if you suspect something might be wrong and use it a guide that your parameters are consistent in the right area if you are adjusting water to suit the fish.
