I normally test nitrates on a weekly basis. No choice due to my heavy feeding. And also because I'm using AZ NO3 to keep my nitrates down, I also monitor to see if I should do a double dose if the nitrates suddenly shoot up. Normally I try to maintain my nitrates between 10-25ppm. Normally when it hits 50ppm i will dose AZ NO3 in the morning and at night and the nitrates will drop back to between 10-25 within 2-3 days.
I seldom test Phophates but when I see an algae bloom, then I will use a phosphate remover like PO4 minus to remove the phosphates. I should probably set up a FR to run an additional PO4 removal media. Right now, I am using carbon and phosphate removal media in my cannister filter.
I also seldom test calcium and magnesium. Basically because I do not keep SPS, the calcium usage of my tank is pretty low. I just dose additives weekly or bi-weekly. When I do a water change using NSW, then I will test all parameters after changing the water.