I test mine every Saturday morning before doing a water change. Once I've seen the results I decide how bigger change to do. I then test again the next day.
Dont really test very often....mostly rely on appearance and smell of water.... Tanks fully mature, so just run the water tests, and i do my water changes by feel so no realy problems: If you can see through it it aint hard enough . Even breeding panda tank i dont test that often, its just knowing your tanks. Test if something goes wrong or something is not right.
6 months ago probably last time, 3 times in the last 18 months according to my log.
I needn't have bothered testing even then- I knew my lack of water changes around the last time would be building up trouble. Didn't lose a fish, just did 2 * 35% changes in a week and all was well again.
Testing every water change etc is pointless really unless you suspect you might have a problem or have an immature/cycling tank. It's just lining the pockets of the test kit manufacturers.