I use it every time I mess with my water, which is every few days. I know my plants keep my nitrate levels in check, but I know my water is in balance by checking my TDS levels are where they should be.
I bought a TDS meter just out of curiosity. In seeing what I could do with it, I started logging the rate and amount of nutrient buildup in my tanks, which was shockingly high.
While this didn't tell me explicitly which nutrients were building up, by process of elimination only a few could be responsible.
This in turn caused me to reevaluate my long-standing belief that my test kits were faulty and giving impossibly high readings. After verification a few other ways, I found they were in fact accurate. And telling me that between fish food and standard EI dosing, my nitrates were peaking at 140ppm, and phosphates at 30ppm, just before each water change.
Realizing that I had more than enough nitrate and phosphate from fish food alone, I completely eliminated both from my fert dosing. Which eliminated the problems with algae and stunted plants I'd been struggling with.