Test intervals are determined by :
a) Age of tank (as this has a direct influence on stability
b) Parameter being tested
c) Maturity of biological filtration system
d) Your dosing and water change regime.
e) Your familiarity of your system and what exactly is happening chemically and biologically.
I have used almost every brand of test kit available locally, and have settled on the following:
1.) Nitrite: Seachem Nitrite and nitrate kit. For a new tank test it every second day to watch your biological filter mature.
2.) Nitrate: Seachem, and Salifert. The accuracy on both these tests are good enough, and once you get down to the part of the scale where the colours are difficult to differentiate, it does not really matter, as you are close enough.
Test weekly once your tank has cycled, and once you have reached the bottom end of the scale (3ppm to 0ppm), measure every second month, just to make sure your dsb for example is still doing its job)
3.) Calcium: A close call between seachem and Lamotte. (I use both, but will have to choose the Seachem, as the end point is more clearly defined, and less open to interpretation, which can be severely influwnced by ambient lightinhg conditions)
Testing is purely dependant on your bioload. In a fowlr I would not even bother. In a sps dominated reef, whilst determining consumption rate and maintenance dose, or dialling in a calcium reactor, I would test every couple of days. Once my supplimentation scheme is dialled in, I would test Alk once a week, and calcium only every second week.(remember ca and alk are consumed in a balanced ratio.........., Well.....almost
)
4.) Alkalinity. Once again a close call between Lamotte and Seachem, but Lamotte would be my first choice. The end point is clear, and refill reagents are cheaper than a new seachem kit. See Calcium for testing protocol,
5.) Magnesium: Test once every few weeks to a month, just to confirm my dosing regimen is still adequate. Seachem wins hands down here. I have tried salifert, and was quite happy with it, but availability is an issue. I have used the tropic marin (ca and Mg combo) was never happy with it, as difficult to determine exact endpoints and ambient lighting too influential.
6.) PO4: Unless you are running sps dominated systems, tweaking colours and willing to fork out close to R1000 for a decent kit like the Deltec/Merck kit, or R2000 for a photometer, to enable you to meausre down to the levels our systems are upposed to run at, DON'T bother. Rather spend the money in ways to combat and export PO4.
If you are obsessed with playing with test kits, the seachem kit is pretty acceptable. An exact number is difficult to determine (my obsession), but is does give you a very good indication of what is happening in your system or when to change PO4 media.
Hope I have not left one out!!
But then again, like Sunburst said, experience is the best kit.