Hi sic0198,
I see you've been here on the forum much longer than me and so may have established tanks or may be starting up new tanks.. anyway, hard to know from the one sentence question what the context is, which might allow those of us that hang out in the beginner section to either give some helpful info or to say we too are not sure.
For instance, if you're in the very early stages of a fishless cycle, I might say to only test once daily. If you were in the middle and later stages of fishless cycling I might recommend testing twice daily, 12 hours apart. If you were in a "fish-in" cycling situation I might also recommend the twice daily thing.
For established tanks where you're confident of a good working biofilter, your testing might have dropped down to almost nothing and you might not test until you see something from your fish or tank that makes you wonder a bit, then you'd test. Under those conditions, one might test at the weekly water change or at the monthly filter change.
Or when adding new fish, one might want to increase to daily testing, just to watch for a mini-cycling condition from the biofilter. And then, when everything looks ok, that could taper off again to not testing so often.
Or you might just be asking the question we all wonder about, which is "Should I test at some frequency even if my fish look great and I suspect nothing is wrong?" That's always just a hard question to answer.
So anyway, the context of the question matters a lot... let us know!
~~waterdrop~~