The calcium content of my fill water is 40. I have no scaling or clouding. There's no source of calcium going into the pool. There's no reason to test for it under those circumstances.

Should people with vinyl pools run a calcium test to get a baseline? Sure. Should we all know the chems of our fill water? Absolutely.

But the question was "how often do you test" and for calcium hardness in my vinyl pool, that would be 'never'.

Would you like to guess what chemical the pool stores want to sell me more than any other on the few occasions when I've had my water tested there?