Well I would assume given how often they are used I think calibrating once a year is good enough. I guess we all do things kinda differently, but I can't remember my last nitrate test. You can always debate the accuracy of the test along with peoples ability to accurately read the tests. I truly despise my API nitrate kit, as someone who gets 20ppm out of the tap the tanks read quite a bit higher. Once beyond 20ppm the chart just goes red, reddish, redder, more red. I personally can not figure out a accurate reading from this, my estimate is always 35-45ppm for my tanks thats as accurate I can read it.
Like someone already mentioned test kits kinda fade away once you have been in the hobby for a while. You should always have one on hand, but mine at least barely get used. As far as calibrating if you have store that tests you water for free using liquid tests you can always compare your readings to theirs to see if both get similar results. I check the accuracy of mine when ever we have a well water test done. I simply split the sample and test with my kit and the other half goes to the lab. The lab is always going to report accurate results, usually much more accurate then I can read my tests. According to their last test my tap water has 24ppm of nitrates. My nitrate kit reads close enough to this number to satisfy me.
Test kits are normally calibrated fairly well, at least as well as we need them in this hobby. As a chem student I understand assuming it is accurately calibrated does not mean it really is. Test kits do also go bad, I had a GH kit do this recently. Would not change colors for me at all, even testing RO water and adding enough drops to reach 12* GH and still no change, so it ended up in the trash. That is generally how I calibrate most of my test. Test some RO, test some water with a sprinkle of KNO3, and one with a pinch. If the kit reports reasonable results (zero or low/moderate/high) its good enough for me.